* Set touch scroll timeout for Grid on GridDragSource

Fixes #9121

* Allow differentiating touch scroll from DnD

Provide Escalator API to check if a touch scroll is active, and to
specify a delay after which a non-moving touch should not scroll.

* Refactor Html5 DnD client side based on API review

- Moved classes to "correct" packages.
- Removed method DragSourceExtension.clearDataTransferText()
- Remove ButtonDragSource in favor of more generic API

* Poll server for changes at the end of each file upload

First step of mobile DND support.

- Add mobile html5 dnd support using polyfill
- Adds a switch for enabling mobile html5 dnd support
- Adds polyfill only when needed
- Ignore native Android Chrome drag start because doesn't work properly
(no dragend event fired)
- Add documentation on enabling mobile HTML5 DnD support
- Add mention of drag-drop-polyfill license
- Fixed issue in polyfill when not using "snapback"
- Add mention of forked polyfill

Fixes #9174

Fixes #8891

Fixes #9292

Fixes the remaining issue from #3944 for Firefox so that a click on the checkbox label calls the focus listener.

Fixes #9290

Reset HDC when encountering unexpected changes in the data.

Additionally this patch fixes an issue with client and server caches
getting out of sync during resets.

Doesn't fix #9261, drag image missing on Safari when dragging grid row because
that has position: absolute and offset.

Makes subclasses of Grid call custom key and mouse event handlers.

Adds two variants:
- find widget of any type
- accept all or only exact matches

Updates uses of findWidget(e, null) in non-compatibility packages.

- Always set some drag data
- Set the dropEffect on dragEnter and dragOver events on drop target
- Send the dropEffect to server on drop event with disclaimer of current support
- Remove _dragOverCriteria_ and use _dropCriteria_ for `dragenter`, `dragover` and `drop` criteria

Tested manually basic DnD and Grid DnD on Mac with Chrome, Firefox, Safari.
Safari is still missing drag image (regression).

Tested manually basic DnD and Grid Dnd on Windows IE11 and Edge.
Drop event for both is still not working properly #9174.

Logic for finding elements corresponding a server-side classname does
not work with inherited classes. For example making MyGrid extends Grid,
you could not find the specific MyGrid, but only the common Grid.

In most cases this is not a problem since these components are usually
the only instance of said superclass, but the Composite introduced in
the same UI, which makes testing them impossible.

This patch adds the specific classname information for ServerConnectors
that can be used to find the correct connector instance.

This patch introduces a Tree component, implemented as a Composite
using TreeGrid. As initial version of the component, some functionality
of the old Tree component has been left out for now.

Partly addresses #8617

Fixes a race condition when expanding multiple items.
Only one expand or collapse request should be sent from
the client before waiting for a response, otherwise the
indexing in subsequent requests will be incorrect.

Adds API to collapse and expand multiple items from the
server, reducing the amount of round trips with multiple
item expands and collapses.

HierarchyMapper now correctly keeps expanded nodes expanded
if their parent is collapsed.

* Set transfer data only when not null (#9178)

Fixes #8892

Note that IE and Edge are not covered by this change - see #8977

* Fix race condition between rendering Grid and setting up headers
* Remove Grid default headers when initializing the widget
* Fix header clean up, add null check to select all checkbox

Failing to do this might cause constructors and listeners in a Polymer
element too be called too early in browsers which need the V1 polyfill.

Refactors AbstractGridKeyEvent, AbstractGridMouseEvent and their
descendants to follow the pattern used in other GWT DomEvents.

Fixes #7633

This fixes problems with profiling the initial paint:
"SEVERE: Got end event for leftoversDownload.runAsync but is currently in null"

Otherwise, the initial row height state update partly negates
an earlier optimization for unnecessary column width calculation.

A composite is included in the server side hierarchy and in the connector
hierarchy on the client side but does not have its own widget or DOM.

To ensure that captions etc are renderer correctly for the root contents,
the client side connector returns both the widget and state for the content
connector.

Server side API related to width and height are automatically forwarded to
the root component to enable easy use of the composite inside different
layout configurations.

Other server side API inherited from AbstractComponent is unwanted, should be
optional and therefore throw an exception by default.

Resolves #2458

* Indicate when multiple rows are dragged (#8398)

Closes #8700

Resolves #8932

Generate drop target style names on every dragenter to make sure they match the primary style name.

Fixes #9058

Fixes #1149

Fixes #8901

This is what the javadoc promises and what DependencyLoader relies on