Completion: Only show properties when initializing Objects with properites

[ErikReider]

Atm everything except for the class properties are visible. In the example below, the only completions that should be shown should be the class properties (prop1 and prop2)

This is how omnisharp (C# LSP) handles this:

Example: https://docs.elementary.io/develop/writing-apps/code-style#initialize-objects-with-properites

[Prince781]

This is tricky to implement because we have to be aware of the surrounding expression. The Vala parser is currently not able to deal with this very well, so we would have to implement something in Vls.SymbolExtractor.