Support overlay formats other than kml, native format

kml is a very rich and complex descritpiton language, only a small fraction of its functionality is meaningful for OruxMaps. Apparently the complexity of parsing kml files causes memory problems at the level of the xml library.

Hence it would be great if oruxmaps would support some simpler format that can be implemented without the overhead of full xml parsing. Any number of formats are more or less fully convertible with gpsbabel.