Include an abstraction for verified apps

And implement it for flathub and snap.

Draft because:

• it could make sense to have a specific icon for it Andy says it's fine
• it would be interesting to think of other places we might want to use the information We can think beyond this MR
• not super stoked with the tooltip approach Alternative is nothing
• needs https://github.com/snapcore/snapd-glib/pull/143 for snap
• needs https://github.com/ximion/appstream/pull/591 for flatpak