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. :^)

Applications/FontEditor/FontEditor.h

@@ -18,6 +18,7 @@ private:
     GlyphMapWidget* m_glyph_map_widget { nullptr };
     GlyphEditorWidget* m_glyph_editor_widget { nullptr };
     GTextBox* m_name_textbox { nullptr };
+    GTextBox* m_path_textbox { nullptr };
 
     String m_path;
 };