Gee New DualKeyMap proposal

[Daniel Espinosa <esodan gmail com>]

I've added a new object called SerializableDualKeyMap to GXml implementing GXml.Serializable interface, and other one called SerializableTreeMap.

Both implement Serializable, allowing you to create XML representation of the objects (if they are Serializable too) contained in a Gee.TreeMap and a new class to manage objects with two keys.

May you want to add the implementation of SerializableDualKeyMap to Gee.

If so, may be would be a good idea to create an Interface called DualKeyMap an abstract AbstractDualKeyMap and a class called HashDuaKeyMap, following Gee conventions, of course implementing the required interfaces.

Code is located at:

https://git.gnome.org/browse/gxml/commit/?h=serialization&id=e25c4831733fa8d28c0f2b540f4536fe77801311

What do you think?

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