* Drop invisible SemanticsNodes from tree

A node is invisible if it is outside of the bounds of the screen and if it is not merged into its (partially) visible parent.

Also in this PR: only set `wasAffectedByClip` to true if the nodes has actually been clipped.

* Fix other failing tests

* renaming

* review feedback

* more doc

* ++

* Allow unmerging of SemanticsNodes

* test passing

* ++

* remove prints

* doc comments

* rectify comment

* review comments

Diagnostics object is.

* Refactor SemanticsData and SemanticsNode to use Diagnosticable.

* Switch toStringDeep to take named parameters.

* Semantic hit test order for Slivers

* ++

* fix double space

* refactoring

* fix test

* comment fix

* tests

* remove accidental debugDumpSemanticsTree

* review comments

* fix button

* refactor to ensureAction + some related doc fixes

* Update docs for markNeedsSemanticsUpdate

* rewording

* rewording

* ensureAction test

* ensureAction test

* ensureAction test

* more tests

* refactor to allways reset node

* tiny fixes

* more test

* doc fixes

* one more test

* review comments

* 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