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/qmltaskmanager.cpp

@@ -47,9 +47,7 @@ using namespace Utils;
 namespace QmlJSEditor {
 namespace Internal {
 
-QmlTaskManager::QmlTaskManager(QObject *parent) :
-    QObject(parent),
-    m_updatingSemantic(false)
+QmlTaskManager::QmlTaskManager()
 {
     // displaying results incrementally leads to flickering
 //    connect(&m_messageCollector, &QFutureWatcherBase::resultsReadyAt,