Add abilities.xml describing the magic spells

This gives the Mana client the spell information it needs for its new Spells window and for casting spells from the shortcut bar: the chat invocation, mana cost, school, description, target mode and required skill levels of each player-castable spell.

The values mirror the spell scripts in serverdata/world/map/npc/magic/ and need to be kept in sync with them.

See mana/mana!277 for the client change relying on this file.