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/HTML/HTMLInputElement.h

@@ -48,6 +48,8 @@ public:
 
     bool enabled() const;
 
+    void did_click_button(Badge<LayoutButton>);
+
 private:
     bool m_checked { false };
 };