This adds support for $id in dropdown conditions, using the same
method used for supporting referencedColumn.id, and extends a test
to exercise the variable. Without this, the dropdown editor gives
an error if $id or rec.id is used, stating that the column is invalid.
Summary:
Dropdown conditions let you specify a predicate formula that's used to filter
choices and references in their respective autocomplete dropdown menus.
Test Plan: Python and browser tests (WIP).
Reviewers: jarek, paulfitz
Reviewed By: jarek
Subscribers: dsagal, paulfitz
Differential Revision: https://phab.getgrist.com/D4235
