Switch Completion code in the editor to Asynchronous

[RobertvanderHulst]

For better performance we should switch to Asynchronous Completion code. Also:

• (when needed in Asynchronous) delay lookup 'when typing' until a certain amount of time has passed (configurable)
• add views to the intellisense database to quickly lookup types and functions that start with a certain name
• I suspect we have to add the Functions for types in external functions classes to the intellisense database as well to be able to include them in the completionlist by scanning the database
• After these changes the code completion 'on type' should:
  • include locals
  • include keywords
  • scan the database for fields, types, namespaces and functions