[FIX] web: modifiers computed when x2many ready

Modifiers (eg. required, readonly, ... on a field) are computed after a
record has been changed. When loading a record they could be computed
based on x2many in a inconsistent state.

eg. on res.partner form, by defaut the email is required depending on:
        [('user_ids','!=', [])]
    but when switching from one record to another this was computed
    wrongly based on the difference between the two records.

This change make the x2many when loading a record ready to be used as
soon as it is set to solve this issue.

forward-port: not needed after saas-15 thanks to framework views rewrite

fix #18239
opw-749483
closes #18815