(core) Polish Record Cards

Summary:
Improvements
 - Widget and column descriptions are now copied when duplicating a table.
 - A Grist Plugin API command to open a Record Card is now available.
 - New Card widgets set initial settings based on those used by their table's
 Record Card.

Fixes
 - Opening a reference in a Record Card from a Raw Data popup now opens
 the correct reference.

Test Plan: Browser and python tests.

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D4164

app/client/ui2018/cssVars.ts

@@ -137,6 +137,7 @@ export const vars = {
 
   /* Z indexes */
   insertColumnLineZIndex: new CustomProp('insert-column-line-z-index', '20'),
+  popupSectionBackdropZIndex: new CustomProp('popup-section-backdrop-z-index', '100'),
   menuZIndex: new CustomProp('menu-z-index', '999'),
   modalZIndex: new CustomProp('modal-z-index', '999'),
   onboardingBackdropZIndex: new CustomProp('onboarding-backdrop-z-index', '999'),

app/client/ui2018/search.ts

@@ -119,7 +119,7 @@ const cssOptions = styled('div', `
   position: absolute;
   right: 0;
   top: 48px;
-  z-index: 1;
+  z-index: ${vars.menuZIndex};
   padding: 2px 4px;
   overflow: hidden;
   white-space: nowrap;