[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 isn't covered for ad-hoc views (which was going away but is coming back just for the git-extension as a private method for the history view)