LibGUI: Add DialogButton convenience class

DialogButton is a small convenience class, that just gives a button to
be used in dialogs for a more consistent look around the system.

Userland/Libraries/LibGUI/Forward.h

@@ -23,6 +23,7 @@ class CheckBox;
 class ComboBox;
 class Command;
 class CommandPalette;
+class DialogButton;
 class DragEvent;
 class DropEvent;
 class EditingEngine;