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
(grafted from 6f199f6ec408ffec15cc202067e2347702d8fe0c)

--HG--
branch : 2.6
extra : source : 6f199f6ec408ffec15cc202067e2347702d8fe0c
Set None instead of False when moving record on top

issue3912
review14291002
(grafted from 0439ac9001836e87780e4631c4498952485df4a9)

--HG--
branch : 2.6
extra : source : 0439ac9001836e87780e4631c4498952485df4a9
Some widgets are registered many times

This could lead to have buttons clicked many times because the clicked event
will be registered many time.

issue3924
review8371002
(grafted from d97ea070448917d64af20fcb043455a8e1adf526)

--HG--
branch : 2.6
extra : source : d97ea070448917d64af20fcb043455a8e1adf526
Detect correctly when to parse value from DateTime, Date or Time field

issue3957
review12341002
(grafted from 50820068cef9807e8775ae8e2768f7ab80395d5f)

--HG--
branch : 2.6
extra : source : 50820068cef9807e8775ae8e2768f7ab80395d5f