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/qmljshoverhandler.h

@@ -56,8 +56,6 @@ class QmlJSTextEditorWidget;
 
 namespace Internal {
 
-class SemanticInfo;
-
 class HoverHandler : public TextEditor::BaseHoverHandler
 {
     Q_OBJECT