Fix eval_leaf when field equals 0

Only None must be considered as a value not set which will be forced by the
domain inversion.

issue3896
review14271002
Set button readonly when record or group is readonly

issue3904
review13301002
Use a timestamp for storing cache of tree states

issue3888
review9281002
Skip relation field for on_change value of One2Many

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

issue3907
review13311002
O2M.set_client should compare with active records

issue3914
review14281004