screenshot: Do not wrongly enable window button

screenshot: Do not wrongly enable window button

The window button is disabled when
 - there are no windows
 - we are in screen-recording mode
 - the session mode doesn't allow windows

However the last condition is only taken into account when
opening the dialog, but not when switching from recording-
to screenshot mode.

Address this by updating the button's sensitivity in a separate
function, so the different conditions are considered consistently.

Closes https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/6990

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2944>