[FEATURE] new edit type: date/time edit

This adds a new edit type for date and/or time editing.

For the sake of clarity, I did not implemented a different display format from the field format. Otherwise, configuration would have been a bit complex. But this prevent the user from being able to display the day of the week in the widget. What do you think?

Also, I suggest to modifiy QgsEditorWidgetWrapper::initWidget to retun a bool. If it returns false (i.e. the provided widget in the UI form has not the correct type), then we can emit a message in the log saying it was badly configured (which layer, which field, which type, what widget given). 
That would be very useful to the power user I believe.