Skip missing key of Dictionary field

issue3944
review5361002
Don't guess the values for no reference

No reference could use '' or None as selection value for the model. So to
prevent to detect false-positive modification and to set invalid value, the
widget must find which one is used as empty model.

issue3933
review11291002
Fix simplify for simple domains ['AND'], ['OR']

issue3955
review9411002
Call on_change's after setting default values

issue3923
review10351002