Marble has support for parsing KML TimeSpan and TimeStamp elements. The KML spec restricts where they can appear. Google has added gx:TimeSpan and gx:TimeStamp elements in the gx:namespace which behave the same like the KML counterparts, but are allowed to appear in other places as well. Add tag handlers for these two gx: tags and respect their appearance (parent elements) according to https://developers.google.com/kml/documentation/kmlreference

Add test cases for both tags.