Add convenience function to set up panel items.

The 'xf-panel-item' custom elements allows configuration of the
indicators and buttons by setting attributes on the element or
by changing properties on the JS object referencing it.

Our feedback panels have a number of standard types, e.g. a
progress panel, summary panel, activity complete panel, etc.

This CL adds a convenience method setPanelType() that sets
up the various panel types using a single argument that can
be used by host elements that embed 'xf-panel-item'.

Bug: 947388
Change-Id: I29199a2402f1f508a4c59891b60891dfccba9cb1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1657078Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Alex Danilo <adanilo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#669140}

ui/file_manager/file_manager/foreground/elements/files_visual_signals.css → ui/file_manager/file_manager/foreground/elements/files_xf_elements.css

@@ -2,6 +2,7 @@
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file. */
 
+/* TODO(crbug.com/947388) revisit the 'xf-' prefix naming for better search. */
 xf-display-panel {
     max-width: 400px;
 }