GtkWindow: Avoid an early exit

When gtk_window_set_titlebar (win, NULL) is called, we were taking
an early exit and forgot to re-map the window. This does not normally
happen in practice, but glade is about to get a 'csd' switch which
lets one toggle back and forth between titlebar and no titlebar.
Adwaita: dimmed backdrop foreground color

To make focused window more evident.
Adwaita: use $backdrop_fg_color where it should

.background:backdrop was not using that one so some labels didn't
look right in backdrop.
Adwaita: darkened a tiny bit bright variant background

Trying to differentiate a bit more the focused window from the
backdrop ones.