A bunch of little fixes

* Got GUIButton "clicked" handler working.
* Changed GUI::Menu so it no longer derives from GUI::Pane, and started
a really simple implementation.
* Fixed doxyfile to ignore Impl objects (used by the Pimpl idiom), and
fixed .gitignore so it doesn't try to store all the Doxygen docs into
Git.
* Fixed GUIScrollBar to be properly namespaced.