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.

platform/core/src/services/ViewportGridService/ViewportGridService.ts

@@ -31,6 +31,7 @@ class ViewportGridService extends PubSubService {
     reset: resetImplementation,
     onModeExit: onModeExitImplementation,
     set: setImplementation,
+    getNumViewportPanes: getNumViewportPanesImplementation,
   }): void {
     if (getStateImplementation) {
       this.serviceImplementation._getState = getStateImplementation;
@@ -62,6 +63,9 @@ class ViewportGridService extends PubSubService {
     if (setImplementation) {
       this.serviceImplementation._set = setImplementation;
     }
+    if (getNumViewportPanesImplementation) {
+      this.serviceImplementation._getNumViewportPanes = getNumViewportPanesImplementation;
+    }
   }
 
   public setActiveViewportIndex(index) {
@@ -122,6 +126,10 @@ class ViewportGridService extends PubSubService {
   public set(state) {
     this.serviceImplementation._set(state);
   }
+
+  public getNumViewportPanes() {
+    return this.serviceImplementation._getNumViewportPanes();
+  }
 }
 
 export default ViewportGridService;