Update the splitter implementation to support a stretch factor and a
collapse flag.
Update splitter example with new features
Update the wx splitter implementation.

This fixes several issues with the default Wx implementation with
regards to allocating space for the split items. Several private
methods were overridden, but such is life. The best long term
solution is to re-implement this widget from scratch, but this
should suffice for at least a while.
Add a context menu example