implement style selection, visitor side

The changes allow a visitor to select one of the installed style
of the Frogsystem 2, instead of seeing the "active style" set in
the admin CP. This is implemented by setting a cookie on the user's
end, so the user's browser has to accept cookies for this feature.

Still missing (but coming soon in one of the next commits):
- possibility to undo selection (i.e. change back to standard)
- template administration scripts for style selection
possibility to clear style selection
template administration for style selection
Merge branch 'alix7' of github.com:mrgrain/Frogsystem-2 into alix7