[XML Markers] Export XML

[AsherGlick]

The generated classes need a way to export a a file as XML. This should be done by creating a new virtual function of

string as_xml()

on the parseable class. Then implementing it on each of the subclasses.

For Icon and Trail the function should return the entire node along with all of the attributes it uses. Unused attributes should NOT be exported as attributes.

For MarkerCategory the function should export itself as a node, and if it has children then it should also export them.