Improve code completion

The IDE usability could be improved with more sophisticated code completion. The following could be implemented:

• support styled code completion proposals (implemented in 0b24a24f)
• function parameters (initially implemented in fca9874f)
• class members (initially implemented in d3d7d290)
• function and class member completion for nested classes (implemented in 37946a16)
• const/var type suggestions
• many more context sensitive suggestions