[FIX] account: 'currency_id' field not defined on account.bank.statement object (correct name is 'currency')
[FIX] web: get conditional user defaults

This is possible to set field conditional defaults, if the field has the attribute "change_default".
The defaults are set by the web client, by calling the method "get_defaults" of ir.values model, when the onchange is triggered on the field on which the condition is.

In 7.0, all onchanges were triggered clientside, one by one. On creation, the defaults of default_get method were pushed in the form, and then, as the values of the fields were changed (from null to the default value), all onchanges on which there was default value were triggered.

In 8.0, onchanges are performed server side, all at once. On creation, the onchange method is triggered by default (wether or not there is a default value for them), for all fields (widget param of method do_onchange of view_forms js is undefined, meaning the onchange is not triggered on a specific field, but on all fields). In such a case, we must call the get_defaults method of ir.values model for all fields (having change_default attribute), in order the conditional defaults to be set in the form view.
[FIX] purchase: replace widget selection by option no_create

For the same reasons as 37c805f26f7a536dc16b6427ba1cd0abbfce85bd
[FIX] ensure default addon paths are valid (existing)

User-provided addons paths are checked for existence (and rejected), but
default addons paths are not checked, and blow up when trying to listdir
them (e.g. when http.py tries to load modules).

This is an issue when using Odoo from the distributed tarballs, because
the packaging currently moves all modules to openerp/addons and removes
the root ("main") addons directory.