Make custom widget self contained

[reubensjohn]

Allow child widget definitions within a Wodget EDL, enable definitions of the js fucntions used within the EDL of the widget. THis way I can just copy the EDL file ( may be in future even export this EDL file with some tags so that can be used across multiple projects by just importing a path to this widget). Today it is a pain, to use a custom widget, I need to copy the EDL of the widget between projects and in addition copy any js scripts it use and also copy other custom widgets that are forced to define outside of this widget.