Add xdg-desktop-portal config

[serebit]

Description

Defaults to xdg-desktop-portal-gtk for all portals. Resolves #436. xdg-desktop-portal v1.17.0 isn't stable, so this PR is untested.

Submitter Checklist

• [x] Squashed commits with git rebase -i (if needed)
• [ ] Built budgie-desktop and verified that the patch worked (if needed)