refactor form field value handling, to get a nicer api and an easier algorithm to get field's value
Details:
* new .typed_value / .display_value on fields
* droped form_field_value on form
* .value attribute of field instead of .initial
* nicer field's __init__, allowing to give a lambda as value's value
[MESSAGES CONTROL]
disable-msg = C0301
[VARIABLES]
# Enable / disable this checker
enable-variables = yes
additional-builtins = _, display_name
[BASIC]
required-attributes=
attr-rgx = (w|[a-z_][a-z0-9_]{2,30})
variable-rgx = (w|[a-z_][a-z0-9_]{2,30})$
argument-rgx = (w|[a-z_][a-z0-9_]{2,30})$
[TYPECHECK]
generated-members=debug,info,notice,warning,error,critical,exception