Various task related improvements.

* Errors are reported back to task users via Failure signals.
* Lwjgl doesn't download on each legacy instance start anymore.
* Tasks were unified when it comes to success/failure.
* Task dialogs don't get spawned after short tasks finish anymore.

backend/LegacyUpdate.h

@@ -47,6 +47,9 @@ private:
 	// MinecraftVersion *targetVersion;
 	QString lwjglURL;
 	QString lwjglVersion;
+	
+	QString lwjglTargetPath;
+	QString lwjglNativesPath;
 };