[IMP] fields: do not force required=True on related fields

This may cause issues if the field is stored: non-null constraint will prevent
saving records when no value is provided.
[FIX] web: revert c1d48362d3a38606ffb99d569fdc6147dcd0c775 and consider there is a previous dialog only if isDestroyed returns False
[IMP] ir_qweb: remove the option 'only_date' introducted in cefc9ad8e3e3b0a14804da6c40bfd71e16d4175a. Now we can use widget='date' to do the same thing. That keeps the same behaviour that in the backend.

    <span t-field=model.datetime_field t-field-options="{'widget': 'date'}"/>
[FIX] packaging: include RML & xsl in manifest.in
[FIX] web: re-enable test_menu.py without the mock-menu test
[FIX] website_sale: better fiscal position detection + support update during checkout

The customer can change the country and tax
number in the billing information during
checkout, and the taxes should be properly
updated according to the re-detected fiscal
position.

The fiscal position detection also depends
on the `vat_subjected` flag, which we now assume
to be implicit as soon as the customer filled
in a valid Tax Identification Number.
[IMP] models: do not use compute methods to determine default values anymore

Compute methods could give results that should not be considered as default
values.  For instance, a related field usually defaults to a null value, which
is then set to the field with its inverse method by create().  This may violate
a non-null constraint if the original field is required.  Therefore, compute
methods are no longer used to determine default values.