scrollbar: update style properties on GtkRange at init

scrollbar: update style properties on GtkRange at init

GtkScrollbar used to rely on style-updated being emitted every time
after the widget was created in order to set the right values from its
style properties on GtkRange.
Nowadays we try to be smarter and avoid emitting style-updated at
creation time, so we need to manually initialize the GtkRange values.

This fixes a regression from 35e36b9fe506143f722a00f26a86143f9c95b740.

https://bugzilla.gnome.org/show_bug.cgi?id=686280