tabwin: Comply with the primary monitor setting

If the user selected a primary monitor, the Alt+Tab window will only
appear on that monitor, otherwise on all of them.

To quote the GDK documentation for gdk_display_get_primary_monitor:

> [...], specialized desktop applications such as panels should place
> themselves on the primary monitor.

It is however not possible to use gdk_display_get_primary_monitor and
also respect the user's setting. Despite what the online documentation
says, recent versions of GTK+3 may no longer return a null value and
will instead pick one monitor as the primary if there isn't already one
defined. The nullable marker [1] for the return value was eventually
dropped.

In other words, whether the user selected a primary monitor or not
gdk_display_get_primary_monitor will always return one.

Signed-off-by: Dridi Boukelmoune <dridi@fedoraproject.org>

[1] https://gitlab.gnome.org/GNOME/gtk/commit/76d95c312dde