`RenderSemanticsGestureHandler` is no longer a semantics boundary, which allows us to correctly mark disabled buttons as disabled without having their semantics size and semantics node id change unexpectedly.

Fixes https://github.com/flutter/flutter/issues/12589.
Fixes https://github.com/flutter/flutter/issues/11991.
See also https://github.com/flutter/flutter/issues/11993.

This change also required some refactoring to how we deal with `twoPaneSemantics` scrolling as it previously relied on `RenderSemanticsGestureHandler` being a semantics boundary. This should also make the underlying logic easier to understand.

In addition, the following minor changes are included in this PR:
* Removal of orphaned and unused `SemanticsConfiguration.isMergingDescendantsIntoOneNode`.
* Logic optimizations for `markNeedsSemanticsUpdate` .
* Fix for edge case where `MergeSemantics` failed to merge semantics.
* Use of emojis to better indicate leaf merging in the printed semantics tree.
* Better assert message for adding invisible child semantics nodes.
* Make some semantics tests robuster by not relying on creation order of SemanticsNode ids across test boundaries.

Fixes https://github.com/flutter/flutter/issues/13943.

* Without checking compatibility

* ++

* ++

* ++

* more docs

* Oct 12 10:12am

* implicit_semantics_test.dart passes

* refactoring

* works in nice

* minor rename

* more doc comments

* to be explicit check better

* fix test

* ++

* ++

* semantics_9_test (BlockSemantics) and implicit_semantics_test are passing

* doc updates

* tiny refactor

* fix static errors in tests

* fix gesture detector

* ++

* ++

* geometry

* ++

* remove noGeometry

* revert test

* +

* all tests but scrolling/clipping pass

* clipping works

* scrolling halfway

* sliver tests pass

* ALL TESTS PASS

* SemanticsNode changed

* docs and tiny fixes

* card test

* more doc comments

* remove missed print

* more tests

* make test pass on Linux

* remove changes to intellij proj file

* review comments

Fixes #11855

Fixes #11381

* upgrade to linter-0.1.35

* re-enable lint prefer_const_constructors

* address review comments

* Exclude SemanticNodes that are fully covered by an overlapping sliver

* simplification

* formatting

* nits

* add test for center widget

* Semantic hit test order for Slivers

* ++

* fix double space

* refactoring

* fix test

* comment fix

* tests

* remove accidental debugDumpSemanticsTree

* review comments

* fix button

* refactor to assembleSemanticNode

* ++

* cleanup

* fix test

* add note

* review comments

* review feedback

* import fix

* another import fix

* refactor to ensure tag

* tests, tests, tests

* analyzer fixes

* review comments