New exercise type: picture description

[heatherleaf]

Instead of making a translation, the exercise could be to create a sentence that describes a picture (or animation)

• the lesson grammar describes simple images, 
or animations that exemplify actions
• the goal is shown as a picture/animation
• the current picture/animation can also be shown
• this doesn't need a 2nd language, but instead 
a graphical interpretation

[heatherleaf]

Depends on #99