core: Add MediaContainer.get_writable()
core: Add MediaContainer.get_writable()

A new method that fetches a GLib.File object for any writable URI
available for the container.

Pelagicore/Media-Manager-Rygel-patched,GNOME/rygel,luisbg/rygel,GNOME/rygel,Pelagicore/Media-Manager-Rygel-patched,Tarnyko/rygel,jku/rygel,Pelagicore/Media-Manager-Rygel-patched,jku/rygel,GNOME/rygel,Tarnyko/rygel