New code assist API
This is a re-work of our completion engine. Primary goals are: - Allow the computation to run in a separate thread so the GUI is not locked. - Support a model-based approach. QStrings are still needed (filtering, etc), but internal structures are free to use more efficient representations. - Unifiy all kinds of *assist* into a more reusable and extensible framework. - Remove unnecessary dependencies on the text editor so we have more generic and easily "plugable" components (still things to be resolved).