The Add Tracks button is a popover button configured to display a list
of playlists that tracks could be added to. I take some extra care to
make sure we only display playlists that have their user-tracks property
set to True, and to hide the currently visible playlist from the list.
Additionally, I create a horizontal size group so the Add and Remove
buttons are the same size.
Signed-off-by: Anna Schumaker <Anna@NoWheyCreamery.com>