gtkwindow: Simplify GTK grab notification

gtkwindow: Simplify GTK grab notification

The _gtk_widget_grab_notify() function just (maybe) did a) reset
controllers and b) hide toplevels. The second part was a testing
remnant introduced in commit 024d832d943, not part of the original
fix.

Do the former more concisely, called from the place where we figure
out whether a widget's ability to receive events changed due to
GTK grabs. It's across those changes that we are interested in
resetting the controllers.

With the gestures being reset both ways, GtkWindowHandle (and
probably other) gestures are now able to reset after a GTK grab
takes input away (e.g. GtkMenuButton). This could be seen as
a sudden jump the next time they'd be dragged with the mouse,
as the gesture would "resume" the previous interaction.

Fixes: https://gitlab.gnome.org/GNOME/gtk/-/issues/3942