now_playing_mpris: added toggle play callback
now_playing_mrpis: added next and previous buttons
now_playing_mpris: switched to icons for buttons
now_playing_mpris: moved toggle button to middle
now_playing_mpris: toggle button now changes icon
Merge remote-tracking branch '5paceManSpiff/mpris-toggle' into develop
now_playing_mpris: fixed widget hiding

After quiting an mpris client, the widget attempts to hide.  This was
previously not working because the "container" object had not yet been
defined.