Added get_min/max_size() methods to ABCUIElement...

useful if an element is changing behaviour dynamically for either dimension
WIP Add split container
Augment and refactor min/max size of split container
Add properties to access min/max width/height for SplitContainer...

This geometry stuff will probably be refactored later, but for now it just
needs to work.
Add tests for VerticalSplitContainer and fix code
Add split containers top level __init__
Added split container example
Merge branch 'split_container'

Add horizontal and vertical split containers. They can take an element
weighting to adjust proportions of the split, and they can handle their
containees having geometry restrictions.