cmake: common addons depends buildsystem

* bfc48a3 adds a new cmake project xbmc-addons-depends which builds all dependencies located in the "common" sub-directory and the platform-specific subdirectory (currently only "windows" for win32).
* af00062 adjusts the win32 build tools to build the new xbmc-addons-depends cmake project (which implicitly builds xbmc-addons-depends-windows) instead of xbmc-addons-depends-win32.
* 9201ca2 moves the logic to copy the addon binding headers and the xbmc-config.cmake file to the prefix directory into a common dependency called "xbmc" and removes that logic from the xbmc-addons CMakeLists.txt

I've only tested this on win32 and any depends based platform (linux, android, darwin, ...) probably needs an additional depends target that calls the new xbmc-addons-depends cmake project.
configure will create the file.