Tracker: use the Layout API.

Sorry this commit is so big, but I couldn't figure out how to do this
incrementally without breaking things.

I wasn't able to just merge Aldeck's branch, as it was a partial refactor
of Tracker and didn't just rewrite the UI creation code to use layouts,
and the changes for PM (e.g. addon loading, virtual directories) made it
very hard to merge (it doesn't even compile after an automerge) so rather
than spending time on that, I decided it'd be better to recreate his work.

Miscellaneous notes:
 - This partially cleans up BPoseView & subclasses and BContainerWindow &
   subclasses -- none of the subclasses and child views abuse the parent's
   state, child views, or layout now.
 - I noticed a lot of #ifdef BEOS_VERSION ... gunk in the code. Tracker
   probably didn't build on BeOS just before this commit, and now it
   won't for sure, so I intend to go through and clean that out in the
   near future.

This commit also fixes:
 - enhancement #4496 (make Tracker's navigator use vector icons)
 - a regression that's a side effect of "dynamic_cast<BDeskWindow*>(this)"
   always returning NULL when run in the constructor. I just added a "bool
   isDeskWindow" to BContainerWindow's constructor that is only set to true
   by BDeskWindow.
 - a copy&paste error in VirtualDirectoryPoseView that was passing "uint32
   resizeMode" as "uint32 viewMode". Not sure how that happened.

Thanks to Alexandre for his original branch (it was a very useful
reference), Axel (for some miscellaneous advice & encouragement),
Adrien & Humdinger (for user interface review), and Diver (for user
interface review & testing).