(core) Add custom widget gallery

Summary:
Custom widgets are now shown in a gallery.

The gallery is automatically opened when a new custom widget is
added to a page.

Descriptions, authors, and update times are pulled from the widget
manifest.

Test Plan: Browser tests.

Reviewers: jarek

Reviewed By: jarek

Subscribers: dsagal

Differential Revision: https://phab.getgrist.com/D4309

app/client/lib/koForm.css

@@ -134,6 +134,10 @@ div:hover > .kf_tooltip {
   z-index: 11;
 }
 
+.kf_prompt_content:focus {
+  outline: none;
+}
+
 .kf_draggable {
   display: inline-block;
 }