OverlayLayout for easy widget alignment in parent with uneven padding support.

Here is a new UI layout that allows to align multiple overlaying children inside the parent. This can be used similarly as a FrameLayout in Android. It also supports setting an uneven content padding globally for all children. Maybe it would be better if padding would be property of child constraints to allow different paddings for different childrens but this is not implemented yet. Do you see such feature resonable?

There is also a new OverlayLayout demo in the demo screen menu to try it out:

![overlay_layout](https://f.cloud.github.com/assets/23485/1839090/c918507c-7470-11e3-8025-27e6c329f8dc.png)