Build limit

Added the possibility of specifying the maximum number of units (closes #2529)

Maximum number of specific units the player is allowed to build/train can be set by setting the Buildable -> BuildLimit property, for example:

    Buildable:
        BuildLimit: 1

The player is not allowed to build more units than specified. If they somehow receive additional units of the same type by other means (capturing a building, crates, ...), they can go over this limit, however they won't be able to build additional ones of course.

In this batch the limit is set for Volkov and Tanya, however it could be applied to any other units or building, such as Missile Siloe (issue #2515)