This patch introduces a new (non-visible) `Mounted` class, to be used by the AI instead of `Cavalry`. The advantages of this include:
* cavalry can be used by auras and technology specifically for true cavalry units
* camels, chariots, elephants could be separated from cavalry templates
* mods can introduce zebras, unicorns, pegasi, //whatever// without having to label them with a Cavalry class
* the AI can (and will) now train (Briton) war dogs