Fix the styling for the media player and make it look the same in the notification drawer and on the lockscreen.
Turns the media player widget into a GtkListBoxRow and places it in a GtkListBox so that the styling is inherited from libhandy. This way the same style can also be applied to the notifications without having the css duplicated. Fixes: https://source.puri.sm/Librem5/phosh/-/issues/371 Fixes: https://source.puri.sm/Librem5/phosh/-/issues/370
CC: @tobias.bernard