add directional focus navigation for floating windows

Map left/down to previous and right/up to next in the workspace
stacking order, with wrapping. Does not cross to the tiled layer.

src/tree/float.rs

@@ -483,6 +483,9 @@ impl FloatNode {
     fn update_child_active(self: &Rc<Self>, active: bool) {
         if self.active.replace(active) != active {
             self.schedule_render_titles();
+            if active {
+                self.restack();
+            }
         }
     }