Adding another convenience method to find all directories of a given resource

that contain a certain sub directory.

Also add install prefix + "/share/apps" to the path list for "data" if it isn't already
in there.
This will allow us to look up agents in a more than one directory, getting us a bit closer
to what KDE applications can do with KStandardDirs

svn path=/trunk/KDE/kdepim/akonadi/libakonadi/; revision=703092