Add SplitterSizing functions to Programmer's Toolbox

[vix]

The idea and the code has been originally posted by @nickb here.

This library handles a common need that I handled in a less elegant way as described here.

As suggested by @RobertoBozzolo the original code from Nick would be a great addition to the Programmer's Toolbox.

In this way all the users would benefit froma standard tool, without re-inventing the wheel (as I did)