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();
 }
 

LibGUI/GCheckBox.h

@@ -2,6 +2,7 @@
 
 #include "GWidget.h"
 #include <AK/AKString.h>
+#include <AK/Function.h>
 
 class GCheckBox final : public GWidget {
 public:
@@ -14,6 +15,8 @@ public:
     bool is_checked() const { return m_checked; }
     void set_checked(bool);
 
+    Function<void(GCheckBox&, bool)> on_change;
+
 private:
     virtual void paint_event(GPaintEvent&) override;
     virtual void mousedown_event(GMouseEvent&) override;