RFC: Add GtkBuilder

This adds support for using Gtk builder (i.e. load Glade files)

I am sure that the implementation is not perfect but it works:

    using Gtk
    builder = GtkBuilder("C:\\test.glade");
    win = getObject(builder, "window1", GtkWindow)
    showall(win)

Issues:
 - GtkBuilder should be an GObject
 - Not sure if it can be avoided to explicitely give the type in `getObject`
 - Maybe `getObject` should be accessible as `builder["window1", GtkWindow]`
 - I have no clue how to handle signal connections defined in xml