Autocomplete of methods for given variables

[krystophny]

Fixes #979.

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[krystophny]

The initial version is working. Up to now, it has autocomplete for functions available from the SymbolServer that have methods that exactly match the type of a single first argument. Next steps should be to support

• [ ] User-defined functions in outside SymbolServer database (current scope / Main)
• [ ] Matching functions with multiple arguments

@pfitzseb could you have a look if the general idea and functionality is OK? I won't have much time in the coming days to continue, but maybe this may serve as a basis for further work. Do you think one could merge also such a "minimal version" in case the other features are much harder to realize?