Convert an extension into standalone application

The CMake variable EXTENSION_WC_REVISION is empty ! error is due to building an extension that is not under revision control. You can choose to either build the extension from a git version-controlled folder or specify the extension revision manually as described here.

While obviously developers must always use revision control, Slicer project should probably not enforce this as a mandatory rule. Therefore, I’ve submitted a pull request that changes the build error to just a warning: