(core) Implement 'Print widget' option to print individual view sections.

Summary:
- Supports multi-page printing with some aggressive css overrides.
- Relies on a new function implemented by grist-plugin-api to print a
  multi-page CustomView.
- Renders all rows for printing for scrolly-based views.

Test Plan:
Doesn't seem possible to do a selenium test for printing. Tested
manually on Chrome, Firefox, and Safari.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2635

app/client/ui2018/menus.ts

@@ -45,6 +45,12 @@ const cssMenuElem = styled('div', `
   z-index: 999;
   --weaseljs-selected-background-color: ${vars.primaryBg};
   --weaseljs-menu-item-padding: 8px 24px;
+
+  @media print {
+    & {
+      display: none;
+    }
+  }
 `);
 
 const menuItemStyle = `