Warn user when file is already open in another view

This is for #9031 

**Cases to test:**
a. single-click
b. double-click
c. drag-drop

**No Split** - should *not* see warning
- file not in editor, not in ws
- file in editor, not in ws
- file not in editor, in ws
- file in editor, in ws

**Split** - should *not* see warning
- file not in editor, not in ws
- file in current editor, not in ws
- file not in editor, in current ws
- file in current editor, in current ws

**Split - should see warning**
- file in other editor, not in ws
- file in other editor, in other ws
- file not in editor, in other ws

Need to squash commits before merging

cc @JeffryBooher 
unfortunately not everyone uses this API to open a file and not all paths flow through here.  The `FILE_OPEN` family of commands just open through MVM directly.  `QuickOpen` is one such consumer which used `CMD_ADD_TO_WORKINGSET_AND_OPEN` rather than use `FileViewController` so any file opened via `QuickOpen, Find In Files, etc...`  will not show this warning.