Move focus to the 2nd pane when ⇧ is pressed in the 1st pane. Fixes #1030

This fixes a bug by Apple, but also makes the feature more obvious. (see #1030)


Other fixes:

* Allow ⌘ (and ⇧) to show alternate actions (focus the 2nd pane) if the capslock key is on as well.
* Don't let the 1st pane resign first responder if it's empty (more reliable than only letting the 2nd pane accept first responder if the 1st pane isn't empty  (done in `acceptsFirstResponder`))


Rob - now I see why you were happy for me to get a new Mac… it was so that I could get annoyed by this and fix it for you ;-)