Frogger game: refactor starter code

- refactor engine and starter app code using es2015 class syntax
- add touchpad tap and double-tap methods (with interpretation/translation to character motion stubs)
- add game scenario variability (frogger, alien attack, scrolling fighter etc)
- add icons (space environment etc)
- add engine extension method

Start by locating the engine and starter app referenced in the issue, then map how input, game scenarios, icons, and extension points are currently organized. Clarify the intended scope before changing anything, since the issue lists several broad refactors and new behaviors without naming files or tests. Done means the agreed engine and starter-code refactor, input stubs, scenario variability, icons, and extension method are implemented and verified.

Written by the indexing model from the issue text.