Prefer dealing with GdkEvent pointers and using accessor functions like
gdk_event_get_button().

This is complicated by the fact that some GtkWidget method declarations
still use GdkEventButton pointers, and synthesizing button events pretty
much requires direct GdkEventButton access.  But GDK seems to be nudging
itself toward sealing the GdkEvent union.  Likely to happen in GDK4.

Mainly clean up signal handlers and leave method overrides alone for now.