GroupEditor does not call super()

The helper class `GroupEditor` does not call `super()` in its `__init__()` and so Traits will not properly initialize:
https://github.com/enthought/traitsui/blob/7681a2b8b851394411213e6f1eeda18b1a02062d/traitsui/qt4/ui_panel.py#L1255-L1262
(similarly for wx).

This could be part of the problem for issues involving `enabled_when`, etc not applying to labels or other components (eg. #758) or updating properly (or not, but it definitely and anti-pattern for Traits to do this).