Tab completion seems to be working well now, for two backends.

- For Maxima and Qalculate everything is rather nice, though the code
in CommandEntry could be structered better. For other backends the
longer time needed for fetchCompletions could be a problem, when user
input comes in between.
- This is not the only problem though: A quick test shows that completion
in Sage does not work anymore, because Cantor crashes whenever a completion
is selected. Apparently some expression is deleted too early. Also this
backend (and probably others as well) are not prepared for the reuse of
CompletionObjects.