(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/declarations.d.ts

@@ -68,6 +68,7 @@ declare module "app/client/components/BaseView" {
     public buildTitleControls(): DomArg;
     public getLoadingDonePromise(): Promise<void>;
     public onResize(): void;
+    public prepareToPrint(onOff: boolean): void;
   }
   export = BaseView;
 }