Make legacy controller public

The one that was added by default on all widgets no longer exists on master. It seems an innocuous small piece of API that might be useful for porting though.

Also, GtkWidget::event is finally dropped, in favor of a signal in the legacy controller.

There is one user left of GtkWidget::event though in gtkimcontextxim.c, it used to listen to configure events, so the only additional brokenness is the warning on the non-existent signal. I need to investigate further if that "status window" can be made optional on the client side and thus dropped here, as we do support on-the-spot preedit text nicely.