All backends have completion again, plus minor bugfix.

- Completion works for R and KAlgebra, but it currently cannot find out the
identifier type in these two backends, so there are no automatic parentheses
or similiar. Adding this feature for KAlgebra should be easy, I just don't
know the library. Maybe I'll look into this.
In R one could use the function is.function to identify functions, but maybe
the RServer can do better? Again I havn't had a close look at the code.