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 
Line 588 adds doc to working set for `ACTIVE_PANE`, and now you're testing that's it's *not* in working set for `ACTIVE_PANE` -- that doesn't make sense to me. Should doc be opened first, then added to working set for the pane that it was opened in? Otherwise, can you add a comment explaining how this is the best order to call this code?