[MERGEABLE] Splitview With Image and Custom View Support

https://trello.com/c/A22yIIal/1282-splitview-add-view-provider-mechanics

This PR is can be merged into jeff/splitview-1x2 when ready
* Adds a MainViewFactory for view factories
* Adds support for images
* Adds support for extensible custom views
* Adds support for 1-off views
* Adds a view state cache singleton
* Fixes #6356

I was also a little thrown again by how MainViewManager.addView() takes a File, while Pane.addView() takes a View.  That naming feels confusing.  But if we change the provider to not call Pane.addView() directly, then maybe that API can be more hidden, reducing the potential for confusion...