application: Drop support for non-unique mode in headless instance

gnome-shell started allowing only applications that own certain names on the session bus to interact with the screenshot interface. While the name used by gnome-screenshot is on the allowed list, non-unique instances that are used for the headless mode do not own that name. Fix this by also making the headless instance unique so it will own the name. This however also means that there can only ever be a single instance of gnome-screenshot, regardless of headless or interactive.

Fixes: #179 (closed)