limit clear and crop function to playlist/playlist editor

this will minimize the risk of unwanted clearing of playlist e.g. by
pressing 'c' key accidentaly in other screen than playlist/playlist editor.

src/ncmpcpp.cpp

@@ -1463,7 +1463,7 @@ int main(int argc, char *argv[])
 				myPlaylist->EnableHighlighting();
 			FreeSongList(result);
 		}
-		else if (Keypressed(input, Key.Crop))
+		else if (Keypressed(input, Key.Crop) && myScreen == myPlaylist)
 		{
 			CHECK_PLAYLIST_FOR_FILTERING;
 			if (myPlaylist->Main()->hasSelected())
@@ -1501,6 +1501,8 @@ int main(int argc, char *argv[])
 		}
 		else if (Keypressed(input, Key.Clear))
 		{
+			if (myScreen != myPlaylist && myScreen != myPlaylistEditor)
+				continue;
 			if (myPlaylist->Main()->isFiltered())
 			{
 				ShowMessage("Deleting filtered items...");