Supporting multiple clients

[fullmoon_sulfuras]

Hi there! I'm working on fixing LunarSphere for TBC Classic and wed like to have it working for the three clients: Retail, Classic, and BCC. The Backdrop System Changes seems the most problematic to handle, as every single frame creation will have an extra argument for inheriting from `BackdropTemplate` for Retail and BCC, but not for Classic.

Any elegant way to handle this? I didn't want to pepper the code with `if` on every single frame declaration.

Perhaps defining a `BackdropTemplate` only for the Classic client?

We have some frame templates defined in XML, so I'm guessing we would need to define the template in XML as well (because it's parsed before the Lua code).

Thanks!