Cleanup and implement the rest of MPRIS 2

Based on gnome-music's
Closes #21
Rename sound_menu to mpris
Fix fake client
Move desktop file to data folder
Remove changelog

It is stored on the website, no reason to maintain two copies
Bump version
Initial port to gtk3/gstreamer 1
Port keybinder.
Initial notification_icon conversion.
Set input_purpose
Remove can_focus restrictions.
Fix AppIndicator3 support
Expand search window list
Use glib to get config dir

Drops requirement of python-xdg
Update deps for Debian package
Port to Gtk.Application

Also replace optparse with argparse
Initial work on appmenu