priorities for java alternatives

[Matthias Klose <doko@cs.tu-berlin.de>]

now that non-free java jre's and jdk's are available in non-free, we
should get some agreement about the priorities for the different tools
and environments. some proposals:

- things in main have higher priorities than things in contrib
  and non-free.

- an alternative installed as a "set" of alternatives has higher
  priority than a single tool.

- tools conforming to a higher "java version" have a higher priority
  (unsure if that is necessary).

- ordering of the free runtimes. can we agree on some kind of order?

Matthias