completion: enable multiple completors for different use cases.

Introduce CompletionPolicy enum, use 'TextCompletion' for the new plain
text completion, and 'SemanticCompletion' or 'QuickFixCompletion' for
the existing cases.

Reviewed-by: Erik Verbruggen

src/plugins/texteditor/quickfix.h

@@ -161,6 +161,9 @@ public:
     virtual int startCompletion(TextEditor::ITextEditable *editor);
     virtual void completions(QList<TextEditor::CompletionItem> *completions);
 
+    virtual bool supportsPolicy(TextEditor::CompletionPolicy policy) const
+    { return policy == TextEditor::QuickFixCompletion; }
+
     /// See IQuickFixCollector::fix
     virtual void fix(const TextEditor::CompletionItem &item);