Restore "progressbar_pixbuf" from gpodder.gtkui.draw

This was removed in 34b54e94b8387848d6549611a2ab9670d5307d8b, since it was no
longer used. However, in the next changset we will start using it again.

Although we're about to use it from the extension, it makes a certain amount of
sense to add it back into draw rather than into the extension, since it uses
some other functions from draw, and is also generic in the sense that it could
conceivably be used elsewhere.

--HG--
extra : rebase_source : ef7edc382dfcd2537c2b3b08ded9cf103d13c41b
Gtk status icon: restore download progress bar functionality

The old (gpodder 2) trayicon.py (which was removed in
a198ecc400b340a3a45f51be397cfad9fe80a2da) had the functionality of showing a
download progress bar on the tray icon itself. The new Gtk status icon
extension did not copy over this functionlity. This changeset ports that
functionality over, and is heavily based on that original code.

--HG--
extra : amend_source : b6cd3349db7f54b8e30b86b9d514073cb80a15fd
Gtk status icon: fix unloading and reloading of the extension

--HG--
extra : rebase_source : 0321ab421a6b523a2311c7ebec27ef19597d59d4
extra : __touch-noise__ : 3693677836
extra : amend_source : 85531ad5ffc889210203a2b0cfb37727367f546c