In case of a DictFeat, LabeledFeatWidget._widget doesn't have a value_to_feat method and communication fails #65

Context: I ran start_test_app on an instance of a custom driver which contained a DictFeat and clicked on its set button. The following error shows up after closing the Test Panel:

Traceback (most recent call last):

  File "C:\Anaconda3\lib\site-packages\lantz\ui\widgets.py", line 470, in on_set_clicked
    self._widget.value_to_feat()

AttributeError: 'DictFeatWidget' object has no attribute 'value_to_feat'

Note that there is no runtime error since the value_to_feat method is never called (see Issue "In case of DictFeat, DriverTestWidget.update_on_change() has no effect").

Probable origin: In case of a DictFeat, LabeledFeatWidget._widget is a DictFeatWidget, which doesn't have a value_to_feat method.

Probable fix: add the value_to_feat method to DictFeatWidget

Proposed fix: add to DictFeatWidget

def value_to_feat(self):
        return self._value_widget.value_to_feat()