Inform users when a permission is required for a demo

[sonnyp]

Currently, Workbench is considered as "probably safe" by Flathub and GNOME Software.

The reason it is not considered "Safe" is because we require network and audio permission (via --socket=pulseaudio and --share=network. Those are not strictly required to use Workbench and are only needed for a handful of demos. In addition, we would also like to add a permission for gamepads https://github.com/workbenchdev/demos/pull/88

Eventually there will be portals for all of these things but until then we need static permissions.

Let's

1. remove those permissions so that Workbench is considered safe
2. add a mechanism for demos to advertise which permissions they need - maybe flatpak-finish-args: ["--socket=fallback-x11"] in main.json
3. when a needed permission is not enabled, Workbench should show a dialog on "Run" to explain what this is about and how to enable the permission using flatpak overrides

Since overrides are permanent - it should be a one time thing only anyway.