(core) Fix linking of new records when attachment is the first thing added.

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

app/client/components/GridView.js

@@ -885,7 +885,7 @@ GridView.prototype._getColStyle = function(colIndex) {
 GridView.prototype.domToRowModel = function(elem, elemType) {
   switch (elemType) {
     case selector.COL:
-      return 0;
+      return undefined;
     case selector.ROW: // row > row num: row has record model
       return ko.utils.domData.get(elem.parentNode, 'itemModel');
     case selector.NONE:
@@ -899,7 +899,7 @@ GridView.prototype.domToRowModel = function(elem, elemType) {
 GridView.prototype.domToColModel = function(elem, elemType) {
   switch (elemType) {
     case selector.ROW:
-      return 0;
+      return undefined;
     case selector.NONE:
     case selector.CELL: // cell: .field has col model
     case selector.COL:  // col:  .column_name I think