Minor improvements in the javadoc

(cherry picked from commit fe0686f6ec38b5016a5ed96722e7acd2443f5f12)
Remove obsolete experimental modules

As previously discussed on the dev@ list, these experimental
modules are no longer needed and better served with
alternatives, or with a refocused effort.
Make experimental modules releasable in 6.x and 7.x

Having the experimental modules in one and the same groupid makes it
impossible to maintain the experimental modules in both 6.x and 7.x
branches--they will be unreleasable due to conflicting version numbers
and users will be confused which release is compatible with which
version of Wicket.

By moving the experimental modules into their own wicket version specific
group id, we can release both experimental modules with wicket releases
for the corresponding major branches.