Fixes for kano code mini remix functionality

[cdrapes]

Minor fixes to aid remix functionality in kano code mini.

• Added accessor for content-widgets
• Added onDidRequestNextChallenge to ChallengeBase as used in Briefing, Challenge and Remix.

[TeamKano]

Coverage summary
Statements	29.41%	2004/6813
Branches	20.38%	212/1040
Functions	29.43%	651/2212
Lines	29.29%	1936/6610

From Jenkins: kano-code/PR-1839