Update front page to reflect current status

Not all SciJava software was explicitly mentioned. It was also hard to
read the table of projects and dependencies.

- Projects are now split into two sections: Libraries and Applications.
- Only the Applications have an "Interoperates with" column now since
  libraries by definition are intended to interoperate with whatever
  wants to use them.
- All projects now have icons, which are used to concisely illustrate
  project relationships.
- Added CellProfiler and ImageJ1 to the list of Applications.
- Added SCIFIO and SciJava Common to the list of Libraries.
- Added blurbs about how these projects work together.
Add brief answer to FAQ question
Remove footer

The footer div is still there in case we want to fill it in later. But I
removed the mention of Sphinx since the page has been hacked up since
then, and Sphinx cannot be used to regenerate the page since the
original source doc is not (AFAICT) in the Git repository.