R1:8b550ce2cd8f

Don't allow the middle mouse button to start an inline comment

Summary:
Ref T13216. See PHI985. When you click a line number to start an inline comment, we intend to initiate the action only if you used the left mouse button (desktop) or a touch (tablet/device).

We currently have a `not right` condition for doing this, but it only excludes right clicks, not middle clicks (or other nth-button clicks). The `not right` condition was sligthly easier to write, but use an `is left` condition instead of a `not…