Make it possible to have implementations in different toolkits.

Also, make it possible to share the headers.

OGApplication.h

@@ -1,23 +0,0 @@
-#include <gtk/gtk.h>
-
-#import <ObjFW/ObjFW.h>
-
-@protocol OGApplicationDelegate <OFObject>
-- (void)applicationDidFinishLaunching;
-@optional
-- (void)applicationWillTerminate;
-@end
-
-@interface OGApplication: OFObject <OFApplicationDelegate>
-{
-	id <OFApplicationDelegate> delegate;
-}
-
-+ (void)quit;
-@end
-
-#define OG_APPLICATION_DELEGATE(cls)	\
-	Class				\
-	og_application_delegate() {	\
-		return [cls class];	\
-	}