window: Use subsurfaces for popovers on wayland

On the wayland backend, set up GDK_WINDOW_SUBSURFACE windows
for popovers. In the popover code, the popover-relative-to-parent
calculation had to be tweaked, and it's been made to always prefer
the given popover position, since there's no sizing limitations.

https://bugzilla.gnome.org/show_bug.cgi?id=738891
Updated Russian translation