Layout is now done only when needed in UI.

- A dirty flag in each UI avatar is used as well as layout requests.
- The layout can go from root to leaves when for example the rendering
surface is resized or a request can go up from leaves toward the root
if an element chooses to resize itself.
- AvatarSpace now supports a « subCountChanged() » callback that is
called each time one ore more sub-avatars are added or removed. This
can be used for the layout requests, or more.