Updated row and spacer handling for Escalator.

Main changes:
- Spacers are only maintained and checked for rows that have DOM
representation, and not at all if there is no details generator. This
gives notable performance improvements to some particularly large Grids
- Escalator no longer tries to trim away any rows that don't fit within
the viewport just because a details row gets opened in Grid. This leads
to some increase in simultaneous DOM elements, but simplifies the logic
considerably. For example opening or closing details rows doesn't
require checking the overall content validity beyond the details row
itself anymore, but some repositioning at most. There are also no longer
any orphaned spacers without corresponding DOM rows.
- Spacers are better integrated into the overall position calculations.
- Some public methods that are no longer used by Escalator or have
changed functionality or order of operations. Any extending classes that
tap into row, spacer, or scroll position handling are likely to need
reworking after this update.
- Auto-detecting row height is delayed until Escalator is both attached
and displayed.

Fixes #7527

Fixes #7833

* Rename HierarchicalDataCommunicator#getMapper to getHierarchyMapper

* Make rpc field in Notification private

* Change DropIndexCalculator.ALWAYS_DROP_TO_END to a generic static method

* Move EditorImpl#editRow documentation to the interface level

* Correct GridDragEndEvent, GridDragStartEvent constructor javadocs

* Revert SharedState.registeredEventListeners to a Set

* Rename GridDropTarget dropAllowedOnSortedGridRows

* Rename ColumnState.contentMode to tooltipContentMode

This pathc also switches GridDragger to by default not allow drops on rows when the
grid has been sorted.

* Add new drop mode ON_GRID for GridDropTarget

Also adds a way to not accept drops on rows when the user has sorted the grid.
This way the bad UX can be avoided for showing the drop indicator for the wrong place when the grid has been sorted.
This has not been made default behavior to GridDropTarget since it would change behavior compared to 8.1.
Instead if is triggerable via API in GridDropTarget.

* Refactor sorted grid drop logic to server side

* Block setDropMode calls

Blocking setDropMode set values if the grid has been sorted and drop on
sorted rows is not allowed. The value is used once the grid is not sorted
anymore or the drops are allowed on sorted rows.

* add Grid#Column#get/setAssistiveCaption

* fix test and update doc

* move testGridAssistiveCaption to GridTest

* delete test file

* delete test file

* Delete GridAssistiveCaptionTest.java

* Create GridAssistiveCaptionTest

* Create GridAssistiveCaption

* Rename GridAssistiveCaption to GridAssistiveCaption.java

* Rename GridAssistiveCaptionTest to GridAssistiveCaptionTest.java

* Reformat using eclipse

* Grid column to be sortable when implemented/supported
* Fix GridDeclarativeTest
* Parameterize to Grid<Person>
* Revert Parameterize to Grid<Person>, JDK with generics.
* Assertions for other columns
* Fix test

Fixes #8792

Opening grid editor from server side.
Fixes #8477.
Addressing #8820 will be the user's responsibility as fetching index of item might be slow.

Also includes minor other javadoc and formatting fixes.

And remove unused imports.

Related to #9425.

Fixes #9425

This feature was present in Vaadin 7 but missing from Vaadin 8

Fixes #9483

Fixes #8548

Also makes sure that the drop location and target row return something sensible in drop event.
Clarifies docs on drop location.

- 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.

Resolves #8932

Fixes #8401

Fixes #8931

Fixes #8400

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

Fixes #8396

Closes #8377

Fixes #8667

Fixes #8622 
Fixes #8623

Fix since tags for the server and shared modules and new types in client.

Fixes #8422

Fixes #8371
Fixes #8128

* Migrate v7 Grid tests. Next round.

Fixes #8419

* Add Grid editor events.

Fixes #8202.

* Add scroll methods to serverside grid (#8203)

Added scrollToTop(), scrollToEnd() and scrollTo(int row)

* Fix scrolling to view of opened details (#8203)

Removed dependency for DetailsManagerConnector from GridConnector.
GridConnector now handles one off listeners.

* Rename detailsRefresh to better show that it's one-off.
Add missing copyright header.

* Add "deselect allowed" feature to the Grid.

Fixes #8149

Change-Id: I73201908b537ca0a65106e03c9bad913b59fae9f

Re-added back disappeared commit from 7.7.5 migration PR:

Mark TextField/TextArea as busy when a text change event is pending
(#20469)

Change-Id: I404985ae0be1e7dc65171b610032f8649e700f50