Preliminary dragndrop

For internal drag and drop I'm going to implement a very simple model for
xwt.lib.draggable...

        // useful for overriding to return
        // alternative content - think 'text'
        __content = function() {
            root.dragndroptype = "box";
            return thisbox;
        }

        _Press1 = function() {
            _Enter = function() {
                root.dragndrop = null;
            }

            _Leave = function() {
                root.dragndrop = content;
            }

            root._Release1 = function() {
                root.dragndrop = null;
                _Enter = null;
                _Leave = null;
                root.Release1 = null;
            }
        }

Then, if an item is interested in dragndrop events, it simply does:

_Release1 = function() {
    if ( root.dragndrop and root.dragndroptype == "yippee" ) ... etc