(core) Update UI for formula and column label/id in the right-side panel.

Summary:
- Update styling of label, id, and "derived ID from label" checkbox.
- Implement a label which shows 'Data Column' vs 'Formula Column' vs 'Empty Column',
  and a dropdown with column actions (such as Clear/Convert)
- Implement new formula display in the side-panel, and open the standard
  FormulaEditor when clicked.
- Remove old FieldConfigTab, of which now very little would be used.
- Fix up remaining code that relied on it (RefSelect)

Test Plan: Fixed old tests, added new browser cases, and a case for a new helper function.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2757

app/client/widgets/TextBox.css

@@ -13,21 +13,22 @@
     color: #D0D0D0;
   }
 
-  .formula_field::before, .formula_field_edit::before {
-    content: '=';
+  .formula_field::before, .formula_field_edit::before, .formula_field_sidepane::before {
+    /* based on standard icon styles */
+    content: "";
     position: absolute;
-    left: 2px;
     top: 4px;
-    width: 12px;
-    height: 12px;
-    border-radius: 2px;
-    line-height: 12px;
-    font-family: sans-serif;
-    font-size: 14px;
-    text-align: center;
-    font-weight: bold;
+    left: 2px;
+    display: inline-block;
+    vertical-align: middle;
+    -webkit-mask-repeat: no-repeat;
+    -webkit-mask-position: center;
+    -webkit-mask-size: contain;
+    -webkit-mask-image: var(--icon-FunctionResult);
+    width: 16px;
+    height: 16px;
+    background-color: var(--icon-color, black);
     cursor: pointer;
-    color: white;
   }
 
   .formula_field::before {