Avoid virtual call in WindowManager ctor

/home/elsid/dev/openmw/apps/openmw/mwgui/windowmanagerimp.cpp:244:9: warning: Call to virtual function during construction [clang-analyzer-optin.cplusplus.VirtualCall]
        loadUserFonts();
        ^
/home/elsid/dev/openmw/apps/openmw/mwgui/windowmanagerimp.cpp:244:9: note: This constructor of an object of type 'WindowManager' has not returned when the virtual method was called
/home/elsid/dev/openmw/apps/openmw/mwgui/windowmanagerimp.cpp:244:9: note: Call to virtual function during construction

apps/openmw/mwgui/windowmanagerimp.cpp

@@ -241,7 +241,7 @@ namespace MWGui
         MyGUI::FactoryManager::getInstance().registerFactory<ResourceImageSetPointerFix>("Resource", "ResourceImageSetPointer");
         MyGUI::FactoryManager::getInstance().registerFactory<AutoSizedResourceSkin>("Resource", "AutoSizedResourceSkin");
         MyGUI::ResourceManager::getInstance().load("core.xml");
-        loadUserFonts();
+        WindowManager::loadUserFonts();
 
         bool keyboardNav = Settings::Manager::getBool("keyboard navigation", "GUI");
         mKeyboardNavigation.reset(new KeyboardNavigation());