Draft: src/core/window.c: only unqueue when calling from queue

Hi folks, this is my first change to Mutter here 😄 and I'm requesting a review.

This change addresses the problem described in #1627

I have tested this change based on mutter releases, but I'm not sure what's the best way to setup a Tip-of-Tree test env. Any advice?

Also, I would welcome input from Mutter experts here in case there's something I'm missing.

Thanks very much!