[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

All function "return undefined" by default, so I don't think this needs to be stated for jsdocs.

If so, then there's a  typo: use `@return` (no trailing 's')