actual bugs found:
* backwards_compatibility.lua (undeclared global "helper")
* core.lua (use of undeclared global "helper")
* wml_tags.transform_unit had wrong code to deal with recall_cost
* wrong variable name in cave_map_generator
Added some features:
1) on_event now supports a comma separated list of event names in its first parameter
2) on_event now supports a number as second parameter that specifies in which order the event handlers are called.
3) on_event now passes wesnoth.current.event_context to the handler function, since the handler function will most likeley need it.
