add a designated CBA_Extended_EventHandlers base class

[commy2]

This is an issue @bux encountered when trying to reenable XEH on some static / building objects.
Usually those have XEH disabled, which is done by clearing the CBA_Extended_EventHandlers.

When setting up inheritance of the EventHandlers class, you actually carry over the references to all sub classes defined in that class.

SignAd_Sponsor 's EventHandlers is from Thing. Thing has an empty CBA_Extended_EventHandlers class to disable XEH on random map objects.
But bux was trying to inherit the actual root class which is also called CBA_Extended_EventHandlers and since Arma can only reference base classes by their name, the empty one from Thing was selected instead of the full one on root config.
The only work around currently is to discard the EventHandlers class, which is not ideal.

This can be avoided by adding a differently named base class. CBA_Extended_EventHandlers_base. CBA_Extended_EventHandlers has to stay for backwards compatibility...