[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

This should reject with a proper error code so that a localized user-visible error string can be shown -- I think it should be `FileSystemError.UNSUPPORTED_ENCODING`, since we can only get here when there's no custom viewer and the file extension is a known-binary type.