#2401 gtk: Set type hint for GTK_WINDOW_POPUP windows

[Jiri Techet]

Without setting the hint the popup windows lead to some obscure behavior
on macOS when the main window is full screened, see

https://gitlab.gnome.org/GNOME/gtk/-/issues/5716
https://gitlab.gnome.org/GNOME/gtk/-/merge_requests/5778

while the solution from 5778 might be a sufficient workaround, setting
the hints might be a good idea in general so the window manager knows better
the purpose of these windows.

Attaching a patch implementing this.

[Neil Hodgson]

Committed with [ce64e8].