Add wlrobs plugin

[damianatorrpm]

Please include https://hg.sr.ht/~scoopta/wlrobs plugin so this flatpak works on wlroots/wayfire/sway & co.

[tinywrkb]

This already works in Sway or wlroots based compositor, you just need xdg-desktop-portal-wlr.
Flatpak-builder does not support Mercurial and the auto-generated tarball has a different checksum each time it is downloaded, see bug report.
In order to get this packaged with flatpak-builder you gonna need to maintain a git mirror or cache the tarball here in the Flatpak packaging git repo.
I gave up trying to reason with wlrobs' developer and using the generic XDG Portal API is a better choice, fragmentation is not the way forward.