Bug fixes and API improvements

Dialog
 - auto-append to body if no parent when show is called

Drop
 - track active documents (TODO a little buggy in the timing sometimes)

Grid
 - fix bugs with creating grid table and mutating prototype definitions

List
 - add getValue/setValue

SelectBox
 - add getter for DataSource

Widget
 - fix construction of children in parent scope chain: the parent must be recomputed at each distinct level of the chain in case a new container was created and child classes should add their children to the new container instead of the old one.  Explicitly set def.children to be an array of all parent classes' children, then process accordingly when building