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.

src/data/Jamfile

@@ -1,5 +1,6 @@
 SubDir HAIKU_TOP src data ;
 
+HaikuSubInclude deskbar ;
 HaikuSubInclude etc ;
 HaikuSubInclude keymaps ;
 HaikuSubInclude mime_db ;

src/data/deskbar/Jamfile

@@ -0,0 +1,6 @@
+SubDir HAIKU_TOP src data deskbar ;
+
+local deskbarMenuEntries = <deskbar>menu_entries ;
+MakeLocateCommonPlatform $(deskbarMenuEntries) ;
+File $(deskbarMenuEntries) : menu_entries ;
+SetType $(deskbarMenuEntries) : application/x-vnd.haiku-virtual-directory ;

src/data/deskbar/menu_entries

@@ -0,0 +1,7 @@
+directory /boot/home/config/settings/deskbar/menu
+directory /boot/home/config/non-packaged/data/deskbar/menu
+directory /boot/home/config/data/deskbar/menu
+directory /boot/common/non-packaged/data/deskbar/menu
+directory /boot/common/data/deskbar/menu
+directory /boot/system/data/deskbar/menu
+