range: read and use CSS margin values for the trough component

range: read and use CSS margin values for the trough component

Many themes want to render the trough background/stroke thinner than the
full height/width (which is constructed around the value of the
'slider-width' style property).
Read and apply the CSS margin from the theme on the trough component, so
that themes can make it smaller at their will without the need to
override the render_background, render_frame and render_activity methods
of GtkThemingEngine.

https://bugzilla.gnome.org/show_bug.cgi?id=676196