gtkwindow: windows with a fixed size can shrink

gtkwindow: windows with a fixed size can shrink

One important aspect of non-resizable windows that we need to preserve
is that they shrink when their content requires less size.

Previous changes to allow the default size to be applied to fixed size
windows would have prevented all fixed size windows from shrinking when
their content requires less size.

Allow shrinking for fixed-size windows unless a default size was
specified.

https://bugzilla.gnome.org/show_bug.cgi?id=762974