cmake: support addons without a build step

This adds very basic support for addons that don't have a build step to the cmake based binary addons buildsystem. The result is basically a simply download (from URL or from a GIT repository) and a copy of the downloaded directory to the install location for addons.

This allows us to download addons like the Chorus webinterface so that we don't have to use git submodules which are causing problems from time to time.

It might also work for skins but there we'd have to make some adjustements to the platform-specific build scripts that execute TexturePacker etc. Ideally that whole process could be done as part of the binary addon build process (as a build step) but we can look into that later.