fix crash/freeze when changing skin after installing a new skin addon

This fixes a crash/freeze when trying to switch to a newly installed skin. The problem is/was that with the new settings system, the skin is automatically changed when lookandfeel.skin is changed and there's no need to manually execute ReloadSkin anymore. The difference however is that the automatic execution of ReloadSkin happens in the same thread where the setting is changed and not in the GUI thread.