Remove the "avatar" concept

Only "character" concept is kept for simplicity.
[demo02] update with regrads previous commit
[demo00] update with regards previous commit
Add a "character_create_from_template" function
[demo01] Allow lich to create skeletons
LUA function "character_set_npc"

This allow to declare a character as a NPC. And it instantiate it if necessary.
[demo01] Allow skeleton to be alive !
[demo01] no diagonal move for AI