themes: prefer symbolic icons when available.

This is a first step to make our 2 symbolic themes into one and properly
"announce" them as symbolic through icon naming (which will allow
recoloring according to style colors).
With this CSS style, GTK+ widgets will search for symbolic icon variants
when using the generic name (with "*-symbolic" suffix).

themes/System/gimp.css

@@ -1,6 +1,7 @@
 /* GIMP theme css file */
 
 * {
+    -gtk-icon-style:                   symbolic;
     -GimpDockWindow-default-height:    300;
     -GimpMenuDock-minimal-width:       200;
     -GimpDockWindow-menu-preview-size: button;