Show mouse position when swapping between providers

Store the mousePosition element id in global variable
mxn.mousePositionElementId, if this exists when swapping providers then
call the mousePosition for the new provider too
Google Earth provider changes

Google Earth relies on a callback before any changes are made to the
map. Custom attributes can not be added to Google Earth objects like
mapstraction_marker - perhaps the id of the element added can be used to
link mapstraction and earth elements?
This provider had dependencies on GEarthExtensions and geo, I could find
no way of setting the map bounds without this
Change leaflet mousePosition to TODO instead of throwing an exception

This function gets called automatically when swapping providers