Make GtkFileChooser interface require a GObject

Make GtkFileChooser interface require a GObject

Before all GtkFileChooser implementations had to be a GtkWidget,
but we want to introduce one for native implementations that
is not a widget.

This is technically an ABI break, because some code could rely
on the guarantee that GtkFileChoosers are GtkWidgets and do
unchecked GtkWidget calls. However, that does seem unlikely,
and this has not really been documented anywhere.