textview: Perform tougher handle visibility checks after hiding the popover

textview: Perform tougher handle visibility checks after hiding the popover

The handles might fall outside the visible area, and shouldn't be shown
then. Just call gtk_text_view_update_handles() which will perform these
checks, and keep the handle conveniently hidden.

This was leading to unexpectedly visible handles (and in the
wrong/previous position, the handle code doesn't relocate the widget
it's about to hide) when "select all" was selected in the popover on
a textview needing scrollbars.
and extending the selection beyond the view above and/or below.