Menu api

This adds a api that does that plugins can add menu to minepeon, without
editing the menu.php. this is good because if a new update is added,
then the code line that the plugin added isn't deleted...

Here is a tutorial how to make an plugin using the api:
https://github.com/MineForeman/minepeon-base/wiki/How-to-make-a-plugin-using-the-api


-Mariogrip