popover: Be lenient wrt visibility of popovers too close to widget borders

popover: Be lenient wrt visibility of popovers too close to widget borders

The check used to hide the popover if the pointed area fell partly out of
the widget allocation, textviews now can trigger that with text selections
too close to the visible edge, as a small extra area around is now reserved.

The check has been changed to only hide the popover if the pointed area
falls completely outside the widget allocation.