show duration of light sources in tooltip

I had forgotten light sources disappear after a while, this way at least
you know it's normal !

apps/openmw/mwclass/light.cpp

@@ -169,6 +169,7 @@ namespace MWClass
 
         std::string text;
 
+	text += "\nDuration: " + MWGui::ToolTips::toString(ptr.getClass().getRemainingUsageTime(ptr));
         if (ref->mBase->mData.mWeight != 0)
         {
             text += "\n#{sWeight}: " + MWGui::ToolTips::toString(ref->mBase->mData.mWeight);