[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 don't see the benefit in ending function name with "For" -- I think it should just be `findSuitableFactory(fullPath)`. If you think we're eventually going to need to be able to find a `Factory` for name and filetype, then it should probably be `findSuitableFactoryForPath(fullPath)`.