Further improvements; smart completion for functions in two backends

- Common code for smart completion, and backend specific code for Maxima and
Qalculate. Code for the other backends (as far as I can write it) is to be
added after a minor design change.
- Syntax help after every completion, with better recognition of identifiers.
- minor improvements/fixes
ToDo:
- Replace virtual completeLine with virtual identifierType, moving move
common code to Cantor::CompletionObject.
- Move the syntax information in mayIdentifierContain and
mayIdentifierBeginWith out of CompletionObject? Currently backend-specific
versions of these are not called, because they are used from within the
constructor of CompletionObject. But they do not belong into this class,
anyway.
- Make the completion popup update, not vanish, when a new character is
entered.
- Implement code to identify variables, functions, and keywords in additional
backends.
- Improve mayIdentifierContain and mayIdentifierBeginWith for some backends?