(core) Adds cell context menu

Summary:
 - Brings in a new utility `contextMenu` to open context menu next to the mouse position
 - Use this utility to show a CellContextMenu, that sort of merge cell context menu and column context menu together.
 - Show cell context menu on context click on any grid's cell.
 - Also takes care of showing the row context menu for detail view on a context click that occurs on cells and not only on the row num header as it was the case prior to this diff.
 - task: https://gristlabs.getgrist.com/doc/check-ins/p/5#a1.s9.r1529.c31
 - discussion: https://grist.quip.com/ETGkAroLnc0Y/Cell-Context-Menu

{F40092}

Test Plan: - Adds project test and nbrowser for cell context menu and new cases for the detail row context menu.

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D3237

app/client/ui/AppUI.ts

@@ -30,7 +30,10 @@ export function createAppUI(topAppModel: TopAppModel, appObj: App): IDisposable
   dom.update(document.body, content, {
     // Cancel out bootstrap's overrides.
     style: 'font-family: inherit; font-size: inherit; line-height: inherit;'
-  });
+  },
+  // prevent default context menu to show
+  dom.on('contextmenu', (ev) => ev.preventDefault())
+  );
 
   function dispose() {
     // Return value of dom.maybe() / dom.domComputed() is a pair of markers with a function that