Fix minor bookmark problems/Enhance bookmark functions

Fix minor bookmark problems/Enhance bookmark functions

This fix:

-fixes when the bookmark menu and submenus are displayed and hidden
 in the context menu.
     -'Create Bookmark' should be hidden when tracks are queued in
      the playlist or nothing is currently playing (previously it was
      never hidden)
     -'List Bookmarks' should be hidden if and only if no bookmark
      file exists for the current playlist (previously it was hidden
      if tracks were queued or nothing was playing neither of which
      hinder loading bookmarks)
     -'Bookmarks' main menu should be hidden if both 'Create
      Bookmarks' and 'List Bookmarks' submenus are hidden

-fixes a problem where the 'Bookmark Error' message was not always
 displayed on bookmarking failure

-adds BOOKMARK_USB_CONNECTED return value to the bookmark functions
 to distinguish if the bookmark list was exited due to a USB
 connection.

-fixes other minor logic problems in the bookmarking functions

Change-Id: If6394b2e77f027773a7c94ffdcb52dbb15e2922b
Reviewed-on: http://gerrit.rockbox.org/177
Reviewed-by: Osborne Jacobs <ozziejacks@gmail.com>
Tested-by: Osborne Jacobs <ozziejacks@gmail.com>
Reviewed-by: Jonathan Gordon <rockbox@jdgordon.info>