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 
The "this branch edited to ignore this flag" test that I ran left in this line of code:

```js
        currentPaneId = _getPaneIdForPath(doc.file.fullPath);
```

If that's not needed, then `currentPaneId` will be an unused var and needs to be removed.

Also, this block is dead code and needs to be removed:

```js
        if (currentPaneId) {
            // If the doc is open in another pane then switch to that pane and call open document
            //  which will really just show the view as it has always done we could just 
            //  do pane.showView(doc._masterEditor) in that case but Editor Manager may do some 
            //  state syncing 
            paneId = currentPaneId;
        }
```