Fix bug #3602 (Can't close Quick Docs via Esc after clicking in left-hand
content) -- Make non-scrolling parts of content focusable so focus stays
in widget (where we listen for Esc) rather than bubbling up to some random
part of CM that no one's listening to.
Merge pull request #3603 from adobe/pflynn/fix-docs-lefthand-focus

Fix #3602 (Can't close Quick Docs via Esc after click in left-hand content)