Fix eval_leaf when field equals 0

Only null must be considered as a value not set which will be forced by the
domain inversion.

issue3896
review8321003
Set button readonly when record or group is readonly

issue3904
review9341005
Skip relation field for on_change value of One2Many

This prevents infinite recursion when the parent has many one2many fields.

issue3907
review14281003
One2Many.set_client should compare with active records

issue3914
review14291003
Add missing ok, cancel as local icons
Fix copy of session in reload context
Remove self hack by using binded method
Fix limit computation for bunch loading
Fix test on bunch loading size
Don't set default value for on_change and set id

issue3487