Qonda

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* Fix: Calendar popup shows current month from empty value when using
DateEditDelegate.
* Ignoring KeyError when a cycling dependency between attributes mess with
attribute chain observing logic.

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* Fix: When DateEdit is empty, calendar popup shows current month instead
Sep 1752.
* Fix: Don't process insertion/deletion keypresses in view widgets when there
is no model set
* Change LookupWidget behavior: Mouse clicks and enter key don't erase current
value.

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* SortFilterProxyModel sorts using Python ordering.
* Fix role color handling in QLabels.
* Don't call callable metadata when object is None
* Add setPyModel() method to adapter classes

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* Fix: Define ObservableObject.__update_set attribute in reconstructor method.
* Return/Enter keys advance fields/appends records in TableView, ListView
* Fix: Use self.item_factory instead of self._class in ObjectListAdapter
when appending the first row

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* ObservableObject handles recursion in update notifications when the observed
object and related object have references to each other.
* Adapters now recognizes the flag key when defined in row_meta argument.
* SortFilterProxyModel proxy class added
* Add selectedObject() method to TableView, TreeView and ListView

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* Aggregators ignore None values
* ObjectAdapter doesn't emit a invalid property warn if there are None values
along the attribute path