Dap.Mtp: Main thread for pixbuf handling (bgo#733659)

It looks like Gdk.Pixbuf gets very unhappy if you don't work with it on
the main thread, even property access generates a gui-thread-check
warning.

In this patch, we block while retrieving the artwork, converting it to a
byte array and recording the geometry.

Signed-off-by: Andrés G. Aragoneses <knocte@gmail.com>