refactor: Uses single QML engine for all QML code (#323)

* refactor: Uses single QML engine for all QML code

* fix: Adds missing `QPointer` include

* Parents QML engine instance to plugin instance.

---------

Co-authored-by: Ivan Lebedev <ilebedev@flightpath3d.com>
Co-authored-by: Ivan Lebedev <ilebedev1988@gmail.com>

chat/NavigationPanel.hpp

@@ -5,6 +5,9 @@
 
 #include <coreplugin/inavigationwidgetfactory.h>
 #include <QObject>
+#include <QPointer>
+
+class QQmlEngine;
 
 namespace QodeAssist::Chat {
 
@@ -12,10 +15,13 @@ class NavigationPanel : public Core::INavigationWidgetFactory
 {
     Q_OBJECT
 public:
-    explicit NavigationPanel();
+    explicit NavigationPanel(QQmlEngine* engine);
     ~NavigationPanel();
 
     Core::NavigationView createWidget() override;
+
+private:
+    QPointer<QQmlEngine> m_engine;
 };
 
 } // namespace QodeAssist::Chat