QmlJSPlugin: reorganize setup

Pimpl, remove use of global object pool, cosmetics, ...

Change-Id: I9e1415b07d7ff8e95db0998c87ce7d75ca638a8e
Reviewed-by: Marco Benelli <marco.benelli@qt.io>

src/plugins/qmljstools/qmljstoolssettings.cpp

@@ -46,8 +46,7 @@ const char idKey[] = "QmlJSGlobal";
 
 static SimpleCodeStylePreferences *m_globalCodeStyle = 0;
 
-QmlJSToolsSettings::QmlJSToolsSettings(QObject *parent)
-    : QObject(parent)
+QmlJSToolsSettings::QmlJSToolsSettings()
 {
     QTC_ASSERT(!m_globalCodeStyle, return);