xdg-desktop-portal support

[mecattaf]

Hello canonical,

With miracle-wm nearing its full release, a huge blocker to general adoption is the weak screen sharing support. As mentioned in the project's documentation: "Unfortunately, Mir only has support for v0.5.0 of this software"

This leaves us with two options:

1. Supporting newest xdg-desktop-portal-wlr (v0.7.1) in mir
2. Creating a similar project called `xdg-desktop-portal-mir'. I have seen this approach successful for the creating of xdg-desktop-portal-hyprland and would be happy to give it a try if you can provide me with some pointers.

@mattkae let me know how you would prefer to approach this! I think it's crucial for the Fedora Miracle spin :)

[Fuseteam]

i think if it's just screensharing, wouldn't ext-image-capture-source-v1 and ext-image-copy-capture-v1 be another option?