Make it possible to have implementations in different toolkits.

Also, make it possible to share the headers.

headers/OGApplication.h

@@ -0,0 +1,23 @@
+#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];	\
+	}