Identifier type is fetched asynchronously, no crash for Sage

- CompletionObject looks to be in a good shape, with everything potentially
time consuming being done asynchronously.
- The appropriate versions of mayIdentifierContain() and -BeginWith() are
now called at the construction of CompletionObject.
- (Popup-) Completion seems to behave nicely for Maxima and Qalculate. In
the other backends everything seems to be working at least as good as it
used to.