This is the final contribution to #15368
* Tried to share more documentation in the various BLayoutBuilder classes
* Add missing GridView, GroupView, SpaceLayoutItem
* Also added AbstractLayoutItem, but hide the actual documentation behind
an `INTERNAL` conditional block. This block identifier can be used to
document parts of the API, to then hide them during a regular Doxygen run.
* Do some cleanup on other layout classes; add missing members, etc.
* The actual generated BLayoutBuilder::* html is a mess. I should investigate
this at a later time. Especially the copied members seem to mix type
definitions with member documentation. It is odd. Not unlikely to be a
Doxygen bug.
* The general documentation for the layout system could use an overhaul as
well, but this is for later.
Change-Id: I6db9ef105b4ae6de0f1ebb917f86f8b1c0d4ea2e
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2491
Reviewed-by: waddlesplash <waddlesplash@gmail.com>
