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.

Also removes dependency on junit.framework.TestCase .

* Fix TestBench API imports, move functionality from custom elements

* Fixes to TestBench APIs and JavaDocs

* Merge remote-tracking branch 'origin/master' into 578_tbapi_cleanup

* Fix method name in CheckBoxGroupTest

* Remove unused custom element classes

* Implement getOptions using getOptionElements

* Replace setValue with setSelection in CheckBoxGroupElement

* Rename CheckBoxGroupElement getSelection to getValue

* Fix one last method

* Prevent adding several scrollbar handlers (#19189).

Change-Id: Ib0cc6c6835aab6d263f153362a328bcf2be7bc5c

* Prevent adding several scrollbar handlers (#19189).

* Keep expand ratio for last row/column when reducing grid layout size (#20297)

Change-Id: Iff53a803596f4fc1eae8e4bfa307b9c1f4df961a

* Fixed drag and drop failure when message dragged from email client (#20451)

When dragging message form email client on Windows, item.webkitGetAsEntry()
might return null creating NPE on the client side. Added additional checks
for this situation.

Change-Id: I569f7e6d0d7b137f24be53d1fbce384695ae8c73

* Change expected pre-release version number pattern in publish report

Change-Id: Icdacecc490d2490ea9e262f5c5736c1dece2a89d

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

Change-Id: I404985ae0be1e7dc65171b610032f8649e700f50
# Conflicts:
#	client/src/main/java/com/vaadin/client/ui/VTextField.java
#	uitest/src/main/java/com/vaadin/tests/components/textfield/TextChangeEvents.java

* Fixed touch scrolling issue in Surface and WP devices (#18737)

Fixed by using pointerevents instead of touchevents when the browser is
IE11,
or Edge. Also added touch-action: none; css rules into escalator.css to
prevent
default touch behaviour on IE11 and Edge. Does not affect IE8 to IE10
browsers,
behaviour on those will stay the same as before the fix.

No new unit tests since we do not have automatic touch testing
possibilities yet.
Please test manually with Surface: IE11 and Edge, use for example
uitest:
com.vaadin.tests.components.grid.basics.GridBasicsomponents.grid.basics.GridBasics

Change-Id: Iddbf1852e6ffafc855f749d6f4ebb235ed0f5703

* Add lazy/simple resize mode to Grid (#20108)

Change-Id: I47427efc28c350382dba8c1f50fd332a3f4585e4

# Conflicts:
#	client/src/main/java/com/vaadin/client/connectors/GridConnector.java
#	client/src/main/java/com/vaadin/client/widgets/Grid.java
#	server/src/main/java/com/vaadin/ui/Grid.java
#	shared/src/main/java/com/vaadin/shared/ui/grid/GridState.java
#	themes/src/main/themes/VAADIN/themes/base/grid/grid.scss
#	uitest/src/main/java/com/vaadin/tests/components/grid/basicfeatures/GridBasicFeatures.java

Change-Id: Ieca56121875198ed559a41c143b28926e2695433

* Fix NPE in case some items don't contain all properties of Grid.
This could occur in when parent is a different entity than its children
in hierarchical data.

Change-Id: Icd53b5b5e5544a3680d0cd99702ab78224b2dc08

# Conflicts:
#	server/src/main/java/com/vaadin/data/fieldgroup/FieldGroup.java
#	server/src/main/java/com/vaadin/ui/Grid.java

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

Change-Id: I404985ae0be1e7dc65171b610032f8649e700f50
# Conflicts:
#	client/src/main/java/com/vaadin/client/ui/VTextField.java
#	uitest/src/test/java/com/vaadin/tests/components/textfield/TextChangeEventsTest.java

* Add lazy/simple resize mode to Grid (#20108)

Change-Id: I47427efc28c350382dba8c1f50fd332a3f4585e4

* Removed V8 VTextField unused import, forgotten @RunLocally.

* Don't rely on selenium "sendKeys" behavior.

* Revert "Change expected pre-release version number pattern in publish report"

This reverts commit 8df27b952d.

* Migrate TextField/TextArea patch from 7.7 to master (modern components)

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

Change-Id: I404985ae0be1e7dc65171b610032f8649e700f50