Add a way to refresh custom maps list without exiting the lobby

[Metadorius]

Currently custom map sharing needs to be done manually for a lot of mods using CnCNet due to how MapDB backend works. Sharing the map becomes even more of a headache because every player needs to restart (or rejoin?) the lobby before being able to load the map.

[Rampastring]

A file system watcher keeping an eye on the custom maps folder could be a good way to do this.