Gkr: KeyringProperties: Use Gtk.LockButton

Closes #199 (closed)

In the impl_update call during release_async, I have intentionally set the allowed attribute to false because

If the user has the permission and cannot drop it, the button is hidden.

I think we just want to disable the button for the duration of the async call, not hide it altogether.

This shouldn't cause any problems wrt usability, since a fresh impl_update call is made after the operation which sources the current permission from the keyring.