Make menus reevaluate their clipping when they change

By simply calling this.clip() when inserting or removing items.

Bonus:
* Document that this is what you're supposed to do in ClippableElement
* Use inheritdoc for MenuWidget#addItems and move useful information
  up into GroupElement's documentation
* Complete half-written comment about label fitting
* Fix documentation typos

Bug: 70115
Change-Id: I9eca6c97f8777149594a986179f70541b1f8866b
Merge "Make menus reevaluate their clipping when they change"