Implement basic support for variable-width fonts.

Also add a nice new font called Katica. It's not used anywhere yet but
I'm definitely itching to start using it. :^)

LibGUI/GCheckBox.cpp

@@ -47,6 +47,8 @@ void GCheckBox::set_checked(bool b)
     if (m_checked == b)
         return;
     m_checked = b;
+    if (on_change)
+        on_change(*this, b);
     update();
 }