widget: propagate scale factor upon reparenting a widget

widget: propagate scale factor upon reparenting a widget

When a widget is created, its default scale is the scale of the
primary screen (for instance 2). But once parented to another widget
its scale factor should be the one of its parent (if parented to a
widget on a screen at scale factor 1, it should be 1).

The problem is that we don't emit the notify::scale-factor signal when
reparenting happens.

https://bugzilla.gnome.org/show_bug.cgi?id=776821