gui/qt: fix PLModel::isSupportedAction( ACTION_PLAY, ... )
Prior to this commit, ACTION_PLAY was supported if:
- the index was not associated with the currently playing
item, or;
- the playlist was paused.
In other words, the option would not be enabled if the playlist was in
a stopped state.
This commit changes the associated logic to the following, meaning
that the action is supported:
- when the user has right-clicked on an entity, and;
- the playlist is either stopped or paused, and;
- the associated index is not currently playing.
Usage of PL_LOCK/PL_UNLOCK has also been replaced by vlc_playlist_lock
to aid code-reasoning and maintenance.
Signed-off-by: Francois Cartegnie <fcvlcdev@free.fr>