Various layout improvements in QTI rendering

Rendering improvements in different question types and more:

• Gap text / fill in blank
  • Line height same for lines with and without gaps
  • Highlight answered gaps to have better overview what is already answered
• Hottext
  • Line height same for lines with and without text selections
  • Render options in a button style instead of just text and checkbox
  • Highlight selected elements to have better overview
• Single choice / Multiple choice
  • Add label element so that label text can be clicked to check the checkbox / radio