(core) Context menu for cards.

Summary:
User was not able to delete cards. This patch introduces a context menu for cards, analogous to the one available for rows on a GridView.

Changes:
- Row numbers on a GridView have the same icon as on columns to make context menu more discoverable.
- Context menu for rows and columns, when activated, didn't switch section in rare conditions (i.e. when the section had 2 or more columns selected, one of which had the same rowId as a column in the section that the user switched from).
- Card list layout and a single card layout has the same context menu as in a GridView, available by pressing the context menu button.

Test Plan: Browser tests

Reviewers: dsagal, paulfitz

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2870

app/client/components/DetailView.css

@@ -52,15 +52,32 @@
 }
 
 .detail_row_num {
-  text-align: right;
   font-size: var(--grist-x-small-font-size);
   font-weight: normal;
   color: var(--grist-color-slate);
   padding: 8px;
+  display: flex;
+  align-items: center;
+  justify-content: flex-end;
+}
+
+.detail_row_num .menu_toggle  {
+  margin-left: 0.5rem;
+}
+
+.detail_row_num:hover .menu_toggle,
+.detail_row_num .menu_toggle.weasel-popup-open {
+  color: var(--color-link-default);
+}
+
+/* hide menu on layout editor */
+.detailview_layout_editor .menu_toggle {
+  visibility: hidden !important;
 }
 
 .detail_row_num::before {
   content: "ROW ";
+  margin-right: 2px;
 }
 
 .detail-left.disabled, .detail-right.disabled, .detail-add-btn.disabled {
@@ -200,6 +217,10 @@
   margin-right: -1px;   /* allow labels to overflow into the padding */
 }
 
+.detail_theme_record_compact .menu_toggle {
+  transform: translateY(-1px);
+}
+
 /*** form theme ***/
 
 .detail_theme_field_form {