(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/components/CustomView.ts

@@ -89,6 +89,12 @@ export class CustomView extends Disposable {
     this.autoDispose(this.cursor.rowIndex.subscribe(this._updateCursor));
   }
 
+  public async triggerPrint() {
+    if (!this.isDisposed() && this._rpc) {
+      return await this._rpc.callRemoteFunc("print");
+    }
+  }
+
   private _updateView(dataChange: boolean) {
     if (this.isDisposed()) { return; }
     if (this._rpc) {