st-widget: Introduce widget resource scale

st-widget: Introduce widget resource scale

This commit makes StWidget manage the scale of which its associated
resources should be multiplied with. The resource scale is calculated
by clutter, and is retrieved by clutter_actor_get_resource_scale(). Due
to the resource scale not always being available, the getter may fail,
and the actual widget that draws the content will have to deal with
this situation.

As the resource scale depends on where on the stage the widget is drawn,
the resource scale will in general be available once the widget is
mapped.

https://bugzilla.gnome.org/show_bug.cgi?id=765011