* 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.