Lua: Bind lifetime of LuaEngine

We want to make sure that the LuaEngine is deleted when the plugin
is removed.

We also derive LuaEngine from QObject so we can use it as a guard in
QObject::connect calls.

Change-Id: I6b0568f11bee283f880ede9432b2a9fe83014758
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/lua/luaengine.h

@@ -24,14 +24,20 @@ namespace Lua {
 class LuaEnginePrivate;
 class LuaPluginSpec;
 
+namespace Internal {
+class LuaPlugin;
+}
+
 struct CoroutineState
 {
     bool isMainThread;
 };
 
-class LUA_EXPORT LuaEngine
+class LUA_EXPORT LuaEngine final : public QObject
 {
-private:
+    friend class Internal::LuaPlugin;
+
+protected:
     LuaEngine();
 
 public: