LibCore: Add a forward declaration header

This patch adds <LibCore/Forward.h> and uses it in various places to
shrink the header dependency graph.

Libraries/LibGUI/DragOperation.cpp

@@ -24,9 +24,10 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
-#include <LibGfx/Bitmap.h>
+#include <AK/Badge.h>
 #include <LibGUI/DragOperation.h>
 #include <LibGUI/WindowServerConnection.h>
+#include <LibGfx/Bitmap.h>
 
 namespace GUI {