#21126 - tankqdialog now emits a dialog_closed signal when closed or
when the contained widget is closed.  The child widget is also detached
from the dialog so even if the dialog isn't gc'd the widget can be.
Updated the engine Base class to release created dialogs when they are
closed and re-structured the code to simplify customization in derived
engines.
Merge pull request #27 from shotgunsoftware/tankqdialog_closeevent

#21126 Fixes to tankqdialog & engine to allow created dialogs to clean up and be cleaned up correctly