Summary:
Fixes a bug when in a linked widget, the automatic reference wasn't being set
for a new record if attachment is the first thing that gets added to the
record.
- Move handling of 'setCursorPos' pseudo-command to GristDoc to support cross-section switching (relevant when moving attachment into a cell of a non-active page widget)
- Modernize code for AttachmentsWidget slightly (better typings, css conventions)
- Change the fix in https://phab.getgrist.com/D3796 from using isolate to using different z-index values, to avoid a change in the look of the cursor on Attachment cells.
Test Plan: Added a test case for what's possible to test with webdriver.
Reviewers: georgegevoian
Reviewed By: georgegevoian
Differential Revision: https://phab.getgrist.com/D3811
