ScreenLayout

Josh,

This is my first pull request, so let me know if anything is off.

The class I would like you to consider for contribution is "ScreenLayout", for lack of a better name. Duplicating the concept from PanelScreen, I wanted Screen functionality that had layout capabilities such as AnchorLayout. 

This may also solve this issue: https://github.com/joshtynjala/feathers/issues/597, so you don't have to add functionality to PanelScreen that contradicts it's intended purpose.

Thanks,
Jerry