wayland: Allow grabless xdg_popups

wayland: Allow grabless xdg_popups

xdg_shell v6 allows grabless popups, whose behavior is not that
different from override redirect windows with no grab to take
keyboard input (and pointer events outside).

This means we can relax the requirement to have a grab before
creating an xdg_popup. The warning is still useful to have so
people stop relying on gdk_window_show();gdk_device_grab() being
an ok pattern to popup a window, it's been moved to wayland
implementation of gdk_device_grab() instead, so we warn if trying
to grab a GDK_WINDOW_TEMP window that's already visible.

https://bugzilla.gnome.org/show_bug.cgi?id=771694