[bug#49972] [PATCH] gnu: xdg-desktop-portal-gtk: Update to 1.8.0.

[John Kehayias]

Updated to 1.10.0 as part of https://issues.guix.gnu.org/51100

Note that I also propagated xdg-desktop-portal as this fixes the bug I 
mentioned earlier in this thread. I didn't see a response from upstream about 
better handling of XDG_DESKTOP_PORTAL_DIR but exporting it only once in our 
packages fixes the problem.