(core) Adding selection layer on top of field element

Summary: Adding a new layer in front of the field_clip element that fixes selected cells' color.

Test Plan: Existing tests

Reviewers: georgegevoian

Reviewed By: georgegevoian

Differential Revision: https://phab.getgrist.com/D3620

app/client/components/GridView.js

@@ -1266,6 +1266,7 @@ GridView.prototype.buildDom = function() {
 
             kd.toggleClass('selected', isSelected),
             fieldBuilder.buildDomWithCursor(row, isCellActive, isCellSelected),
+            dom('div.field_selection')
           );
         })
       )