Bring the changes that aren't package management related and the ones
that are but don't take effect as long as they are ignored by the build
system into the master.
Summary of changes:
* Introduce private header <directories.h> with constants for a good
deal of paths that should usually be retrieved via find_directory().
* Replace hard-coded paths by using find_directory() or the
<directories.h> constants (e.g. in drivers and the kernel).
* Add find_directory() constants needed for package management.
* Add __HAIKU_ABI_NAME and B_HAIKU_ABI_NAME macros.
* src/apps/deskbar: BeMenu.* -> DeskbarMenu.*,
DeskBarUtils.* -> DeskbarUtils.*
* Change deskbar menu settings directory from ~/config/be to
~/config/settings/deskbar.
* Other smaller cleanups, changes, and fixes.
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