MediaPlayer: Fix movement of items in playlist

In Playlist, whenever a move of items occured causing the
currently playing song to change its position, so :
1. Importing files (D&D for example) before its position
2. Removings files before it
3. Moving files before it

was causing the currently playing song to restart because
it was thinking a new entry was asked (it had a different
index number suddently).

Also adjusted the behaviour when you delete the currently
playing track.

Should fix ticket #6689.

src/apps/mediaplayer/MainWin.cpp

@@ -605,6 +605,10 @@ MainWin::MessageReceived(BMessage* msg)
 			BAutolock _(fPlaylist);
 
 			int32 index;
+			// if false, the message was meant to only update the GUI
+			bool play;
+			if (msg->FindBool("play", &play) < B_OK || !play)
+				break;
 			if (msg->FindInt32("index", &index) < B_OK
 				|| index != fPlaylist->CurrentItemIndex())
 				break;