feat(css-hidden): add/remove class "ui-layout-hidden" for hidden containers

If a container has content with absolute positioning and non-zero padding, the
contents of the containers overlap if either of them gets collapsed. This commit
adds a class attribute and CSS style to avoid rendering the hidden elements.

Author: whummer

src/ui-layout.css

@@ -152,3 +152,8 @@
     overflow: visible;
   }
 }
+
+/* Make sure hidden elements are in fact not rendered. */
+.ui-layout-hidden {
+  display: none;
+}

src/ui-layout.js

@@ -928,6 +928,11 @@ angular.module('ui.layout', [])
 
                 scope.$watch('container.size', function(newValue) {
                   element.css(ctrl.sizeProperties.sizeProperty, newValue + 'px');
+                  if(newValue === 0) {
+                    element.addClass('ui-layout-hidden');
+                  } else {
+                    element.removeClass('ui-layout-hidden');
+                  }
                 });
 
                 scope.$watch('container.' + ctrl.sizeProperties.flowProperty, function(newValue) {