QmlJSEditorPlugin: Refactor

This follows the recently introduced pattern for plugin setup
- Pimpl QmlJSEditorPlugin
- remove unneeded uses of global object pool
- apply "static pattern"
- simplify some constructors of data members in some cases
- use in-class initialization in some case

Change-Id: I95b42d0885f4a8d6c9bfe1e4c004d3ace0a3eba5
Reviewed-by: Marco Benelli <marco.benelli@qt.io>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/qmljseditor/qmljsquickfixassist.h

@@ -53,8 +53,8 @@ private:
 class QmlJSQuickFixAssistProvider : public TextEditor::IAssistProvider
 {
 public:
-    QmlJSQuickFixAssistProvider(QObject *parent = nullptr);
-    ~QmlJSQuickFixAssistProvider();
+    QmlJSQuickFixAssistProvider() = default;
+    ~QmlJSQuickFixAssistProvider() = default;
 
     IAssistProvider::RunType runType() const override;
     TextEditor::IAssistProcessor *createProcessor() const override;