wayland/window: Update toplevel main monitor before non-toplevel

When updating the main monitor, make sure to update the toplevel main
monitor before trying to use that as the main monitor for non-toplevel
windows (such as popups). Without this, when the main monitor is
updated as a side effect to monitors being changed (for example due to
a hot plug event, or coming back from being suspended) the
main monitor pointer may, after 'monitors-changed' has completed, point to
freed memory resulting in undefined behaviour.

https://bugzilla.gnome.org/show_bug.cgi?id=784867