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.

* Sanitize input used in Grid header

* Fix duplicate grid column visibility change event

Toggle a column visibility through UI will fire 2 event. There should be only 1.

* Add Test

- limit DataCommunicator workaround from #11320 to ComboBox only
- don't reset DataCommunicator before modifying all sorting data

Fixes #11532

* Code format

* Merge branch 'master' into code-format

Fixes #11451

* Fix an NPE caused by the previous fix to Grid's frozen columns (#10653)

If the user managed to get two frozen column count changes into a single server round-trip before the component had been attached, the previous fix  #11346 lead to a null pointer exception since there was no diff state available.

This fix adds a null check before removing the frozen column count value from the diff state so that this will not happen.

Closes #10653

* add possibility to configure nested null behavior to Grid.Column

* added addColumn method that allows configuring the nested null handling behavior of the column

* added uitest for nested null value handling, implements #11137

In certain cases setting the frozen columns didn't produce the expected result in the client side widget state. This happened if the frozen columns value was set to be the same it was before removeAllColumns was called.

This fix removes the frozen column value from the diff state so that the value gets properly set in the client side.

Closes #10653

Added note that style generator is not applied to the Editor

See also: https://github.com/vaadin/framework/issues/11169

* Updated JavaDoc of getColumn(..) method

Fixes https://github.com/vaadin/framework/issues/9920

This patch reverts the fix #10557 and replaces it with a
proper solution from Grid perspective.

Fixes #10987
Fixes #10985

Fixes #10928

Fixes #9074

* @since 8.3.2

Fixes #10146
Fixes #10543

Fixes #7833

Also update declarative writing to use true instead of "" for empty
attributes because of changed defaults in JSoup. For v7, the tests are
updated instead of touching the implementations.

Deprecate com.vaadin.ui.components.grid.DescriptionGenerator
and subclass it from the new DescriptionGenerator.

* 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 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

* Grid.removeColumn() not to fail silently (Fixes #8056)

* Compilation with JDK

* Fix removeColumnByColumn_alreadyRemoved test

* Use ExpectedException

* Simplify lambda expressions

And remove unneeded 'return' keyword.

* Format

Also includes minor other javadoc and formatting fixes.

Fixes #9944

Fixes #9967