builtin: Move common/async_event.lua to mainmenu/async_event.lua

It only works in the context of the mainmenu.

builtin/mainmenu/async_event.lua

@@ -0,0 +1,32 @@
+
+core.async_jobs = {}
+
+local function handle_job(jobid, serialized_retval)
+	local retval = core.deserialize(serialized_retval)
+	assert(type(core.async_jobs[jobid]) == "function")
+	core.async_jobs[jobid](retval)
+	core.async_jobs[jobid] = nil
+end
+
+core.async_event_handler = handle_job
+
+function core.handle_async(func, parameter, callback)
+	-- Serialize function
+	local serialized_func = string.dump(func)
+
+	assert(serialized_func ~= nil)
+
+	-- Serialize parameters
+	local serialized_param = core.serialize(parameter)
+
+	if serialized_param == nil then
+		return false
+	end
+
+	local jobid = core.do_async_callback(serialized_func, serialized_param)
+
+	core.async_jobs[jobid] = callback
+
+	return true
+end
+