fix(ViewportGrid) fill blank viewports with display sets not yet in grid. (#3154)

* fix(ViewportGrid): In ViewportGrid, fill blank viewports with display sets not yet in grid.
ViewportGridService now allows off-screen viewports to remain so that
so as to maintain continuity if they were filled by the UI/user.

* PR feedback: moved getNumViewportPanes into the ViewportGridService API.

* PR feedback:
- renamed some variables
- proper import/export of IDisplaySet
- added some comments for clarification
- fixed broken e2e tests

* Some missed rename of Services.

extensions/default/src/Toolbar/ToolbarLayoutSelector.tsx

@@ -79,7 +79,15 @@ function LayoutSelector({
         ],
       });
     }
-    viewportGridService.setLayout({ numRows, numCols });
+
+    // When a new layout is selected, keep any extra/offscreen viewports
+    // so that if any of those viewports were populated via the UI then they
+    // will be maintained in case those viewports are redisplayed later.
+    viewportGridService.setLayout({
+      numRows,
+      numCols,
+      keepExtraViewports: true,
+    });
   };
 
   return (