Homepage update

This HTML/CSS update fixes a number of issues at the same time:

1. Patterns with missing icon get a default icon automatically, until a dedicated icon is designed. 
2. It's possible to link to the download or documentation state directly through a URL. (Purely CSS driven)
3. Documentation and download links now work from demo pages. One no longer always ends up directly on the homepage in welcoming state.
4. Firefox support. 
5. Improved bumper behaviour of logo.
6 .Some more Patternicons.

Todo: 
- Design new hero image for homepage. 
- Further improve bumper behaviour of logo (Requires a fix in the bumper pattern to also respect CSS margin displacements)