[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

@peterflynn it's a little more than just calling the `registerViewFactory` API isn't it?  You have to create a view constructor and a view factory object to construct viewers which is why i didn't just point to the other API.  I guess there's no harm in pointing them there though.