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 
yes, so adding a file to the working set may not actually add it to the working set if it's already open in the working set that isn't active so we check to see if was actually added to the working set by searching for it. I'll add a comment explaining that.