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
(grafted from e2ccc0ff4368c3342f300eb5205c2a3fa2fd7df5)

--HG--
branch : 2.8
extra : source : e2ccc0ff4368c3342f300eb5205c2a3fa2fd7df5
Fix simplify for simple domains ['AND'], ['OR']

issue3955
review9411002
(grafted from 90b6308d43a17fc9f0d775a48670277d25162abc)

--HG--
branch : 2.8
extra : source : 90b6308d43a17fc9f0d775a48670277d25162abc
Quote doublequote characters when copying records to clipboard

issue3972
review6341002
(grafted from 52b6005d779a4a7c07291fb6f7628ee8d8c5263e)

--HG--
branch : 2.8
extra : source : 52b6005d779a4a7c07291fb6f7628ee8d8c5263e
Clean many2one only if text was modified

As update_completion emit the signal changed to force the display of the popup
when the completion model is changed. In this case the many2one should not be
cleaned.

issue3881
review7371002
(grafted from 439e92599164af699ba3daca928590d3fd1acd82)

--HG--
branch : 2.8
extra : source : 439e92599164af699ba3daca928590d3fd1acd82