Add C++ API for driver settings

Copied from the userlandfs utility classes, adjusted coding style, did
some renaming and small interface changes.
BPathMonitor: add "from path" to B_ENTRY_MOVED notifications
BPoseView::AttributeChanged(): additional check in icon case

Model::AttrChanged() requires the model to be open and so does
IconCache::IconChanged(), so add the check. Not sure if the block is
ever executed now (or ever worked before). This does at least prevent
asserts being triggered when compiled with debugging enabled.
Add virtual directory feature to Tracker

Similar to stored queries, files of the virtual directory type behave
like directories -- i.e. they open in a list-mode Tracker window and
show up as an item with submenu in navigation menus. The file itself is
a plain text file in driver settings format. It can have an arbitrary
number of "directory" entries, which specify the paths of (actual)
directories for which the virtual directory provides a merged view. The
view will not show duplicate entries. For non-directory entries the
first one encountered (according to the order the directory paths are
specified in the file) will be shown. A subdirectory entry will again
behave like a virtual directory.

The support in Tracker isn't perfect yet. I'm afraid major refactoring
would be necessary to get it there.

The virtual directory file type uses a differently colored version of
the folder icon. Alternatives welcome.
SetType rule: add optional type parameter

If not specified the executable MIME type is used as before.
Deskbar: use a virtual directory for the user-configurable menu

* Deskbar now uses ~/config/settings/deskbar/menu_entries for its menu,
  falling back to /system/data/deskbar/menu_entries, when the former
  doesn't exist. The latter always exists and is a virtual directory
  merging the deskbar/menu subdirectories of ~/config/settings/ and
  <any installation location>/data/. So, if a package provides a
  deskbar menu symlink, it is added automatically when the package is
  activated. The user can add own menu items by putting stuff into
  ~/config/settings/deskbar/menu/, only use their own organization by
  symlinking it to menu_entries, or do fun stuff by making menu_entries
  a customized virtual directory.
* HaikuImage: No longer create any deskbar menu symlinks in the user's
  settings directory. Instead add them to the Haiku package.
* OptionalPackages: At least for the optional packages that do have
  hpkgs, no longer create deskbar menu symlinks in the user's settings
  directory.
* Move all Deskbar settings files to ~/config/settings/deskbar/ and
  drop the "Deskbar_" prefix.