Add a playback OSD menu option to cancel an in-progress playlist.

Previously, the only way to stop playlist playback was to execute a
jump point, as EXIT simply moves to the next playlist item.  Now, the
playback OSD menu, "Playback" submenu, has a "Cancel Playlist" entry,
with the effect that when playback ends, it returns to the Watch
Recordings screen instead of the next playlist entry.

mythtv/libs/libmythtv/tv_play.cpp

@@ -10182,6 +10182,12 @@ void TV::OSDDialogEvent(int result, QString text, QString action)
         PrepareToExitPlayer(actx, __LINE__);
         SetExitPlayer(true, true);
     }
+    else if (action == "CANCELPLAYLIST")
+    {
+        setInPlayList(false);
+        MythEvent xe("CANCEL_PLAYLIST");
+        gCoreContext->dispatch(xe);
+    }
     else if (action == ACTION_JUMPFFWD)
         DoJumpFFWD(actx);
     else if (action == ACTION_JUMPRWND)
@@ -11449,6 +11455,8 @@ void TV::FillOSDMenuPlayback(const PlayerContext *ctx, OSD *osd,
         }
         if (!db_browse_always)
             osd->DialogAddButton(tr("Toggle Browse Mode"), "TOGGLEBROWSE");
+        if (inPlaylist)
+            osd->DialogAddButton(tr("Cancel Playlist"), "CANCELPLAYLIST");
         osd->DialogAddButton(tr("Playback data"),
                              ACTION_TOGGLEOSDDEBUG, false, false);
     }

mythtv/programs/mythfrontend/playbackbox.cpp

@@ -4177,7 +4177,8 @@ void PlaybackBox::customEvent(QEvent *event)
                 m_artTimer[(uint)type]->start(s_artDelay[(uint)type]);
             }
         }
-        else if (message == "EXIT_TO_MENU")
+        else if (message == "EXIT_TO_MENU" ||
+                 message == "CANCEL_PLAYLIST")
         {
             m_playListPlay.clear();
         }