added draggable widget, sortable list widget, example for sortable widge...

Hi,

I have made a sortable list for organising sequential data (e.g. tracks, effects).

The list is controlled by dragging its elements up and down. There is an example in the "example-ofxUIAllWidgets" directory (on gui4).

Do you think this is a useful addition to ofxUI? If so then I am happy to collaborate to help get the branch up to the necessary standard.

Best,

John.