gtkpopover: Move GTK grabs to map/unmap

gtkpopover: Move GTK grabs to map/unmap

Typically, a popover gets mapped when shown and unmapped when
hidden. A situation there that breaks is where the popover gets
recursively unmapped/unrealized when its root is destroyed.

In that situation, the popover does however unmap (without being
hidden first), moving the GTK grab from show/hide to map/unmap
will handle the previous situations, plus this one.

Fixes things being unclickable if e.g. a modal dialog got a
popover popped up, then got closed via Alt-F4.

Closes: https://gitlab.gnome.org/GNOME/gtk/-/issues/5730