window/wayland: Reset position when hiding popup

When a popup is remapped, if the GdkWindow::x/y position is negative, some best-effort hueristics used to ignore internal X11-style hackery windows incorrectly scooped up remapped popup windows.

Avoid these hueristics by resetting the x and y fields of GdkWindow to 0 whe hiding a popup. It'll then appear as if it was newly initialized, and escape the mentioned hueristics.

Closes: #4071