Added methods to delete songs and song groups.

- OsuGroupList.get().deleteSongGroup() will delete a song group from the list.
- OsuGroupList.get().deleteSong() will delete an individual song from the list (must be expanded), and will delete the song group if there are no remaining songs in the group.
- If possible, deleted beatmap files are moved to system trash (using JNA); otherwise, they are deleted immediately.  This is done through Utils.deleteToTrash().

Signed-off-by: Jeffrey Han <itdelatrisu@gmail.com>

src/itdelatrisu/opsu/states/SongMenu.java

@@ -396,6 +396,8 @@ public class SongMenu extends BasicGameState {
 
 				// search produced new list: re-initialize it
 				startNode = focusNode = null;
+				scoreMap = null;
+				focusScores = null;
 				if (OsuGroupList.get().size() > 0) {
 					OsuGroupList.get().init();
 					if (search.getText().isEmpty()) {  // cleared search
@@ -606,6 +608,8 @@ public class SongMenu extends BasicGameState {
 			// reset state and node references
 			MusicController.reset();
 			startNode = focusNode = null;
+			scoreMap = null;
+			focusScores = null;
 			oldFocusNode = null;
 			randomStack = new Stack<SongNode>();
 			songInfo = null;