Gui: Properly reset light directions

This fixes lights not getting reset after user demands so.

src/Gui/PreferencePages/DlgSettingsLightSources.cpp

@@ -327,6 +327,10 @@ void DlgSettingsLightSources::resetSettingsToDefaults()
 {
     PreferencePage::resetSettingsToDefaults();
 
+    hGrp->RemoveASCII("HeadlightDirection");
+    hGrp->RemoveASCII("BacklightDirection");
+    hGrp->RemoveASCII("FillLightDirection");
+
     loadSettings();
     configureViewer();
 }