In Differential, make the "Open in Editor" keystroke work with no selection, or a change or inline selected

Summary:
Ref T13515. Currently, "Open in Editor" only works with a file-level selection.

• If we have a change-level or inline-level selection, open the parent changeset.
• If we have no selection, but the banner is showing something, open the fine shown in the banner.

Test Plan: With files, inlines, changes, and no selection, pressed "\". Saw files pop open in my external editor.

Maniphest Tasks: T13515

Differential Revision: https://secure.phabricator.com/D21148