replaced missing reference to VerticalThreePane and HorizontalThreePane
allow exceptions to propagate past exit from nested blocks; also use
explicit parenting insteadof ".."
Merge branch 'master' of github.com:theodox/mGui
add wrap method to allow mGui wrappers around existing controls
Added 'derive' method and support infrastructure. Derive allows recursive
wrapping of existing gui
added better error msg for nonexistent attribs; array properties try to
return empty lists instead of `None`