gutter: work around recent GdkEvent changes
gutter: work around recent GdkEvent changes

This is a minimal port to work more like the old API, but eventually we
would want to get rid of all of this in favor of widgets per row (once
that can be made fast and recycle widgetry).

GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview