PlaylistsView and MPRIS want to know when playlists become available to
setup some actions. This was previously done by emitting a signal after
the initial load of playlists. However, in certain  conditions this
signal could already have fired before the consumer was even read, so no
initial action would be taken.

For example, PlaylistsView by default selects a row for intial display
when the list is filled. However, without a signal being received, no
row is selected to display.

Now PlaylistsView instead directly listens to the playlists model for
changes and uses an initial state variable to check if it is needed to
activate a row.

Closes: #348