Make StackOrganiser extendable

[mgnslndh]

Add virtual keyword to the Organise method so the class can be extended with custom sorting of the items.

Provide mechanism to force reorganisation when items needs to be sorted. @ButchersBoy suggested calling InvalidateMeasure which I guess would work but it would be nice to have a solution that works with MVVM as well.

@ButchersBoy:

Override Organise and sort the items and pass to the base. Then you could InvalidateMeasure on the control itself....maybe i could add a routed event or something to "invalidate" the current sort which might make it easier in the future.

[ButchersBoy]

Extendable changes made.

I am considering enhancing the position monitor (mentioned in #82) to provide some bi direction communication.

This could become more of a PositionController, am currently weighing this up.