Groups for layouts

Right now layouts can only be applied to UIContainers. But we may want to create groups of assets that live with some spatial relationship to each other but not have to write a container for it (containers are wasteful since they will add a level that needs to participate in the invalidation etc.

Proposal:
Groups are non visual elements that can have associated layouts. Unlike UIContainers, layouts will not calculate sizes on the groups, so an element in a group with a 100% width will still be measured by the layout of the UIContainer. However layouts can be applied to groups to spatially arrange the controls.

Sample code to lay out three children horizontally
bc. var g:Group = new Group();
g.add(child1, child2, child3);
g.layout = new HLayout();