Prevent downloading songs that are already loaded. (base: fluddokt/opsu/76778f8)

- All beatmap set IDs are now stored in a set in OsuGroupList.
- OsuParser now checks the directory name for beatmap set IDs if the OsuFile doesn't contain one (for older beatmap formats).

Tweaks:
- Clear completed downloads when hitting "Import All".
- Call Download.updateReadSoFar() upon starting downloads instead of waiting for user to hover over the button (causing an unnecessary delay).

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

src/itdelatrisu/opsu/downloads/Download.java

@@ -165,6 +165,7 @@ public class Download {
 					rbc = new ReadableByteChannelWrapper(readableByteChannel);
 					fos = fileOutputStream;
 					status = Status.DOWNLOADING;
+					updateReadSoFar();
 					fos.getChannel().transferFrom(rbc, 0, Long.MAX_VALUE);
 					if (status == Status.DOWNLOADING) {  // not interrupted
 						status = Status.COMPLETE;