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.
Until Wayland support becomes a requirement or until GTK+3's minimum acceptable version increases to the point where the new behavior is guaranteed, this can be conditionally solved for X11 via the RandR extension.
Supersedes #350.