app: qmlregister: Revert signature of registerTypes to fix unit tests.

This reverts an (inadvertent?) change made in 70a6972b7 which would
fail the compilation of the tests.

* src/app/qmlregister.h (registerTypes): Change parent type from
MainApplication* to QObject*
* src/app/qmlregister.cpp (registerTypes): Likewise.

Change-Id: I1ef0aff38fcc9e07192d505614ebf9e2aa023921

src/app/qmlregister.cpp

@@ -106,7 +106,7 @@ registerTypes(QQmlEngine* engine,
               AppSettingsManager* settingsManager,
               PreviewEngine* previewEngine,
               ScreenInfo* screenInfo,
-              MainApplication* parent)
+              QObject* parent)
 {
     // setup the adapters (their lifetimes are that of MainApplication)
     auto callAdapter = new CallAdapter(systemTray, lrcInstance, parent);

src/app/qmlregister.h

@@ -67,5 +67,5 @@ void registerTypes(QQmlEngine* engine,
                    AppSettingsManager* appSettingsManager,
                    PreviewEngine* previewEngine,
                    ScreenInfo* screenInfo,
-                   MainApplication* parent);
+                   QObject* parent);
 }