Allow deriving a gamemode from another.

This would open up many possibilities, for instance to make new gamemodes that inherits from sandbox, so they could use some of features from sandbox which may be considered as "standards" (like the toolgun, spawn menu, ...)

There would still be only one gamemode to load (the lowest one in the derivation chain), but before it is loaded, it would look for and load its parent gamemode in the same VM as the child gamemode, so every feature from the parent gamemode could be overriden