LibWeb: Implement <input type=submit> without using LibGUI

Following in the footsteps of <input type=checkbox>, this patch adds
LayoutButton which implements a basic push button using LibGfx styling
primitives.

Libraries/LibWeb/Forward.h

@@ -136,6 +136,8 @@ namespace Web {
 class EventHandler;
 class Frame;
 class LayoutBlock;
+class LayoutButton;
+class LayoutCheckBox;
 class LayoutDocument;
 class LayoutNode;
 class LayoutNodeWithStyle;