updated addon helpers

in particular, we now handle buildsystems which output several add-ons
properly. real life case; screensavers.rsxs
[WIN32] added a xbmc-config.cmake.win32 for windows
[WIN32] added xbmc_find_path to find paths recursively
[WIN32] removed prepare_addon_environment(). this is done in the prepare-env step now.
move to addon helpers. fits better and avoids duplication for win32