Fix inconsistent json attribute name in dynamic.js

All other segments call it "solution" (singular) but gap-fill-text called
it "solutions" plural
Make gaptext work again + handle gaptext solutions

- Introduce a new nesting-level ("parts") in solutions
- Introduce an own class for solutions
- Implement separat rendering and solution-handling methods in GapText
- adapted views and style2.css
- small comment-fix in gaptext.html template
fix issue #35 (desription columns of input fields were droped)