Add wenoth.set_music compatibility function

data/lua/backwards-compatibility.lua

@@ -4,6 +4,10 @@
 
 local helper = wesnoth.require "lua/helper.lua"
 
+function wesnoth.set_music(cfg)
+	wesnoth.wml_actions.music(cfg)
+end
+
 -- Calling wesnoth.fire isn't the same as calling wesnoth.wml_actions[name] due to the passed vconfig userdata
 -- which also provides "constness" of the passed wml object from the point of view of the caller.
 -- So please don't remove since it's not deprecated.