Remove MVM._edit, etc...

This is for #9569 
* Migrate use of `MainViewManager._edit()` in core with `CommandManager.Execute(Commands.FILE_OPEN,...);`
* Implement Open Document when dirtying a document and adding it to the working set of an empty pane
* Fix #9569 

NOTE: `GotoAgent.open()` is experimental code which is disabled and hasn't been thoroughly tested in this respect.  The code change looks sound but we may want to discuss with @dangoor 

CC: @peterflynn 
yeah -- probably a pie in the sky optimization.