Add labels to grab modes

[mcclurgm]

Fixes elementary/portals#106.

Before:

After:

This adds labels to the radio buttons that set the grab mode. I've tried to be as concise as possible, so the window layout can be as similar to the current implementation as possible. The copy may need design. Of course, it's also possible that there needs to be more design work in general, and if we want to add labels we can't keep the original layout at all. I think this is enough, though.

This needs some work though. The selected styling doesn't apply once I add a label to the buttons, and of course I think we will still want that. I'm not sure how to solve this, since it seems like the selected style isn't defined in the screenshot project itself.

[danirabbit]

Closing since the screenshot window is now provided by the portal, sorry for missing this previously!