Allow mpris to play after queue is loaded, and after playlist ends

Sets mpris::can_play(true) after a playlist is first loaded, which allows playback to be started from mpris without first clicking play in the application window. The playback state is also set to 'pause' rather than 'stop' after a playlist ends; this is a workaround for the currently lacking ability of mpris to transition Amberol from the stopped to the playing state.

Closes #342