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).

src/plugins/qmljseditor/qmljseditor.h

@@ -160,6 +160,9 @@ public:
 
     static QVector<QString> highlighterFormatCategories();
 
+    TextEditor::IAssistInterface *createAssistInterface(TextEditor::AssistKind assistKind,
+                                                        TextEditor::AssistReason reason) const;
+
 public slots:
     void forceSemanticRehighlight();
     void followSymbolUnderCursor();