gtk/window: Only fake motion events on windows with pending allocations

gtk/window: Only fake motion events on windows with pending allocations

This fixes an issue where the focus of the window continuously received
fake motion events even when a popover was open, making input events end
up behind the popover.

It also adds a comment describing why motion events are requested. Note
that popovers won't work with this, and it's possible both in the past
and now that sporadic missplaced motion events will appear, e.g. when a
window changes allocation but a popover is open.