Reset peakZ on new print
(cherry picked from commit 83a864d)
Removed peakZ again, doesn't work this way

Slicers might raise the print head first for a "nose dive" onto the print platform, leading to a high peakZ already dialed in, with no way to get it down again. This way events won't be fired until the print reaches the height of the initial starting point. As a z-change is a z-change if z changes, we'll just fire the event now (if oldZ != newZ). Event consumers will have to think of a way to filter out the noise.
(cherry picked from commit 9227bb5)