ThickClient: add "Selected Track(s)" submenu (bgo#726338)

Behavior of the options from the "Edit" menu was not so clear:
did these options target the playing track or the selected ones?
So, adding "Selected Track(s)" submenu and moving all the appropriate
options into it makes it much more clear.

Signed-off-by: Andrés G. Aragoneses <knocte@gmail.com>
Refactoring: rename "SelectedTracks" methods&labels (bgo#726338)

Renamed all "...Tracks" to "SelectedTracks" labels and methods,
following the previous commit, which places all the actions under
this "Selected Track(s)" submenu.

This is especially useful, to distinguish them from the next
methods and actions which will be committed in the next changeset,
which will introduce a new Playing Track submenu.

Signed-off-by: Andrés G. Aragoneses <knocte@gmail.com>
ThickClient: Add "Playing Track" submenu (bgo#726338)

When some user requested to have CoverArt modification for the
playing track in the "Edit" menu (bgo#724795), we realised that
it was not clear at all that this menu affected just to selected
tracks, not to the playing track. The two previous patches then
modified the UI to include these options under a new "Selected
Track(s)" submenu.

Now, this change introduces a similar submenu (with the same
options) but which applies only to the playing track. This only
works for now if the track played is in the main Music library,
and it can be extended for more sources in the future.

Signed-off-by: Andrés G. Aragoneses <knocte@gmail.com>