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

Fixes #8401

Fixes #8931

Fixes #8400

Fixes #8397

It is possible to customize the drag data for each row. (#8706)

Fixes #8396

Fixes #9050

* Make Button component draggable (#9037)

* Add own state to button drag source

* Add server-side expand and collapse to TreeGrid

* Add javadocs

* Fix variable naming in TreeGridHugeTreeTest

* Fix review comments

* Merge remote-tracking branch 'github/master' into 8759-server-expand

* Clear pending expands when all data is dropped

* Add documentation

Fixes #8758

Fixes #9005

Fixes #8945