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
d. quick open

**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 
Hmmmm...That's a good point. I guess it's just unit tests that use that API.