(core) Add new color select to the app

Summary:
 - Fix transparency support on color select
 - Fix z-index conflicts with color select and right panel
 - Makes widget's default text color visible to color select

Test Plan: - Updates nbrowser/CellColor and browser/Widget.test to support new interface. Should not cause regression.

Reviewers: paulfitz, dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2735

app/client/widgets/FieldBuilder.ts

@@ -421,7 +421,7 @@ export class FieldBuilder extends Disposable {
             if (this.isDisposed()) { return null; }   // Work around JS errors during field removal.
             const cellDom = widget ? widget.buildDom(row) : buildErrorDom(row, this.field);
             return dom(cellDom, kd.toggleClass('has_cursor', isActive),
-                       kd.style('--grist-cell-color', this.field.textColor),
+                       kd.style('--grist-cell-color', () => this.field.textColor() || ''),
                        kd.style('--grist-cell-background-color', this.field.fillColor));
           })
          );