Menu items that rely on focused editor should be disabled when not available

[jasonsanjose]

1. Make a text selection in a file
2. Click on the background of the working set (observe the editor selection color changes indicating that focus was lost)
3. Choose Edit > Duplicate from the menu
4. Observe no changes

Pull #2026 changes focusedEditorChange to activeEditorChange. We probably still want a focusedEditorChanged event to know when a blur event happens to CodeMirror. These menu items (probably the file and edit menus) would listen for this new event and disable/enable the commands as necessary.

[peterflynn]

A corollary to this bug: clicking in dead zones like the working set bg shouldn't steal focus from the editor in the first place. It's inconsistent with how native apps work. I believe that's encompassed by the focus management story (which evolved from #1106).

If that's addressed, then this bug only occurs in cases where the focus is somewhere legit, e.g. the Find bar text field.

[njx]

Reviewed. Marking low priority.

[jasonsanjose]

Closing, move to backlog https://trello.com/c/yMTclKIs/435-enable-disable-commands-for-menus