Support XDG Device portal

• I have read the specified guidelines for issues

Is your feature request related to a problem? Please describe.

This issue originated from https://github.com/tenacityteam/tenacity/issues/436.

Since the upcoming Flatpak package has access to the host directory, it'd be really good if we can start using portals to further sandbox Tenacity to increase its security without losing any convenience.

Describe the solution you'd like

Now that we're making a Flatpak package, we should consider using XDG Device portals. When adding support, at least in Flatpak, Tenacity will ask for permissions and then access your devices when granted.

Here's an example with the Decoder application when it tries to access my camera:

If you deny access, you will be able to grant permissions in Flatseal or your DE's permission manager.