Fix crashes in QQuickWebEngineSettings

Remove QQuickWebEngineSettingsPrivate object's pointer from the global
setting list before the object is destructed.

Change-Id: I1cca6bf8aa2438903692d9e2ef863c8fc543adbd
Reviewed-by: Andras Becsi <andras.becsi@digia.com>

src/webengine/api/qquickwebenginesettings.cpp

@@ -78,6 +78,7 @@ QQuickWebEngineSettings *QQuickWebEngineSettings::globalSettings()
 
 QQuickWebEngineSettings::~QQuickWebEngineSettings()
 {
+    allSettings->removeAll(this->d_func());
 }
 
 bool QQuickWebEngineSettings::autoLoadImages() const