implement UiWidgetTree

[hohwille]

We need to properly implement UiWidgetTree. This is quite important as ListTable and Tree are required for a reasonable application and an appropriate showcase.
For GWT:
We still have to consider if we want to support Cell instead of Widget and move to CellTree. We found a design how we could render a widget such as UiWidgetLabel as Cell (TextCell) instead of Label. However, this is a tough decision as Cell does not really fit into the concept of the ui-widget framework. How could the widget hold the reference to the Cell and update its style and other things? How would a composite widget work (as CompositeCell)? This Cell stuff seems to open a parallel universe and we currently fear a maintenance hell when going towards this...

[hohwille]

So far working including lazy loading on expand, etc.
Various features still missing. Most important is selection mode with multi-selection (via checkboxes). Also expand/collapse should be added to the API.