Automatically hide i3bar when it's unneeded.

Automatically hide i3bar when it's unneeded.

When a workspace marked 'urgent', i3bar unhide
itself. if I want to hide it again, I must press the
modifier.This sometimes annoys me.

In this patch I change the above behavior to this:
If a urgent workspace occurs, i3bar will unhide itself;
and when you navigates away from the last urgent
workspace and there is no more urgent workspace, i3bar
will hide itself.