Prusa i2 motor X end with built-in Z endstop activator

This is a Prusa i2 motor X end with a built-in Z endstop activator. It is based on Greg Frost's X end (the Z endstop activator on the version in his [GitHub repository](https://github.com/GregFrost/PrusaMendel) doesn't seem to be usable; I fixed it in this version). This can be used with Greg's idler X end from his repository.