GtkIconHelper: Preload icons for mapped widgets with higher priority

GtkIconHelper: Preload icons for mapped widgets with higher priority

We look at whether a widget will be mapped (the actual state is not
yet set, so we can't rely on that at css validation time) and use
that to set the i/o priority of the async task.

This means that its likely that widgets that will be displayed soon
are loaded before those that are not yet going to be needed.