"Primary Display" and "Contains top bar and Activities" are visible when
you have >1 display connected and the "Join Displays" option is
selected. I can't work out how to see the "Active Display" string but it
sure looks translatable to me.
This introduces a GtkStack to handle the pages; move the
current panel to be the "displays" page; and adds the
Night Light page as "night-light".
A stack switcher was added, as a header widget, to control
the stack.
https://gitlab.gnome.org/GNOME/gnome-control-center/issues/533
Having the instructions inside can be slightly confusing because
monitors cannot be moved around in the labels area. Simply moving the
instructions to be at the top again improves this.
With this change the dialogs UI matches the old layout more closely.
