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 
@redmunds I think "pane" is a pretty common term for UI elements, so it won't be unfamiliar to users.  We should be crisp in our user-facing terminology to distinguish between split-panes and their contents.  In our code, we use the terms "pane" and "view" respectively, and those seem like pretty reasonable choices for user-facing terms too.