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/qmljssemanticinfoupdater.cpp

@@ -125,8 +125,7 @@ QmlJSTools::SemanticInfo SemanticInfoUpdater::makeNewSemanticInfo(const QmlJS::D
     semanticInfo.setRootScopeChain(QSharedPointer<const ScopeChain>(scopeChain));
 
     if (doc->language() == Dialect::Json) {
-        Utils::JsonSchema *schema =
-                QmlJSEditorPlugin::instance()->jsonManager()->schemaForFile(doc->fileName());
+        Utils::JsonSchema *schema = QmlJSEditorPlugin::jsonManager()->schemaForFile(doc->fileName());
         if (schema) {
             JsonCheck jsonChecker(doc);
             semanticInfo.staticAnalysisMessages = jsonChecker(schema);