renamed pre_store -> transform_value, removed dead code
Instance: store path of tree items having an error condition

error_paths or error_messages should be checked once the asynchronous
store are done
typo in comment
added async store

check for store is done in an async way. store no longer returns a value.
during the async store, _pending_store counter is set. any fetch will block
until _pending_store goes back to zero, i.e. all check store are done.
test async store
do not store error_list in check_fetched_value

error_list is set up in check_value. Handling of error and warnings are
now similar
test: fixed mandatory element and boolean
check_value is synchronous without a callback parameter

This allows simple check_value calls that are useful when
setting up default values.
change error_list attribute into errors attribute with Array trait
change warning_list attribute into warnings attribute with Array trait
factorize error checking
remove redundant test
fix store test
async test is all fixed for value.t
use new _store_value method when replace option is set

this method was recently extracted and is used to store the value at the
right place and to trigger notify_change
fix check attribute initialisation
forward check parameter to load_data
rewrote INI parser so that check is enabled directly on store

data are now stored in an intermediate data structure so they
can be written in the tree using the element order of the model
added some intermediate tests display
fixed section_map