(core) Moving attachment's widget icon below the frozen columns

Summary: Attachment icon in cells overlapped other cells in rozen columns.

Test Plan: Manual

Reviewers: paulfitz

Reviewed By: paulfitz

Subscribers: paulfitz

Differential Revision: https://phab.getgrist.com/D3796

app/client/widgets/AttachmentsWidget.css

@@ -2,6 +2,12 @@
   display: none;
 }
 
+.attachment_widget {
+  /* Create a new stacking context, primary for frozen columns which have z-index:1 and the icon
+     inside this widget (with z-index:1) is visible over the frozen column. */
+  isolation: isolate;
+}
+
 .attachment_widget:hover .attachment_hover_icon {
   display: inline;
 }