Fixes #9121

* 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

* Create upload URLs only when stream variable is attached by drop handler

* Add file size limit to test ui and remove label drop target

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 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.

Fixes #9241

- 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.

* Use correct TreeElement and server-side classname
* Re-enable test, remove unnecessary imports

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.

Workaround for issue in data communication where
active keys are dropped incorrectly on the server.

See issue #9217

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.

* 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

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

Fixes #7633

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

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

Fixes #8943

Closes #8377

Fixes #8667

Fixes #8702