(core) Adds scafoldings for the narrow screen layout

Summary:
This commit adds a bottom bar with 2 openers  button for each side panels.
It takes care of showing/hiding other openers when appropriate.
It also implements the new style for narrow screen with the side panels overlapping the main pane.

Remaining tasks:
  - adjust content of the breadcrumbs for narrow screen.
  - content of the bottom bar

Test Plan: Adds test to projects/PagePanels

Reviewers: paulfitz

Reviewed By: paulfitz

Subscribers: dsagal

Differential Revision: https://phab.getgrist.com/D2701

app/client/ui/AppUI.ts

@@ -147,5 +147,6 @@ function pagePanelsDoc(owner: IDisposableOwner, appModel: AppModel, appObj: App)
     contentMain: dom.maybe(pageModel.gristDoc, (gristDoc) => gristDoc.buildDom()),
     onResize,
     testId,
+    optimizeNarrowScreen: true,
   });
 }