Separate engine files

[DavidLeoni]

Engine files and images are polluting the template.

Proposal 1:

img/ada.gif     ->  demo/img/ada.gif
img/play.svg    ->  img/engine/play.svg
img/turtle.svg  ->  img/turtle.svg   

Proposal 2:

img/ada.gif     ->  demo/img/ada.gif
img/play.svg    ->  engine/img/play.svg
img/turtle.svg  ->  img/turtle.svg   

Proposal 3:

img/ada.gif     ->  demo/img/ada.gif
img/play.svg    ->  img/play.svg   
img/turtle.svg  ->  img/turtle.svg