Make additional features more easily accessible

[peterwvj]

Based on the INTO-CPS review, we suggest that Overture features are be made more easily accessible by:

• [x] Mentioning extra features on the Overture welcome page
  • Overture FMU
  • VDM2C
  • Graphics features
• [x] How to install Overture features (Help -> Install new software ...)
• [ ] Update overturetool.org to also describe additional features and how to install them
  • [ ] Short features description of above mentioned features, possibly one page per feature

[peterwvj]

Overture now uses the new welcome page. Strictly speaking the remaining updates only affect the website.