improve on_event.lua

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.

data/lua/on_event.lua

@@ -1,16 +1,44 @@
+local utils = wesnoth.require "lua/wml-utils.lua"
+-- registers an event handler. note that, like all lua variables this is not persitent in savefiles,
+-- so you have to call this function from a toplevel lua tag or from a preload event.
+-- It is also not possible to use this for first_time_only=yes events.
+
 local event_handlers = {}
 local old_on_event = wesnoth.game_events.on_event or function(eventname) end
 wesnoth.game_events.on_event = function(eventname)
 	old_on_event(eventname)
+	local context = nil
 	for k,v in pairs(event_handlers[eventname] or {}) do
-		v()
+		if context == nil then
+			context = wesnoth.current.event_context
+		end
+		v.h(context)
 	end
 end
 
-local function on_event(eventname, handler)
+local function on_event(eventname, arg1, arg2)
+	if string.match(eventname, ",") then
+		for elem in utils.split(eventname or "") do
+			global_events.add_priority_event_handler(elem, arg1, arg2)
+		end
+	end
+	local priority = 0
+	local handler = nil
+	if type(arg1) == "function" then
+		handler = arg1	
+	else
+		priority = arg1
+		handler = arg2
+	end
 	eventname = string.gsub(eventname, " ", "_")
 	event_handlers[eventname] = event_handlers[eventname] or {}
-	table.insert(event_handlers[eventname], handler)
+	table.insert(event_handlers[eventname], { h = handler, p = priority})
+	-- sort it.
+	for i = #event_handlers - 1, 1, -1 do
+		if event_handlers[i].p < event_handlers[i + 1].p then
+			event_handlers[i], event_handlers[i + 1] = event_handlers[i + 1], event_handlers[i]
+		end
+	end
 end
 
 return on_event