mpris: send correct desktop-entry hints

According to spec [1], the ".desktop" suffix must not be present.

[1]: https://developer.gnome.org/notification-spec/
mpris: be recognized by GNOME Shell as a "music" notification
mpris: add standard action buttons

This changes the notification to always be resident, otherwise it would
be dismissed when unpausing.
mpris: move debug message to better place

To have a cleaner diff for the $hints{"image-path"} commit later.
devel/pull: automerge after svnsync/git-svn