Use a decorator to track Z offset

This makes it much easier to correct for Z offset after operations

CURA-196 #Ready-for-Review

cura/ZOffsetDecorator.py

@@ -0,0 +1,13 @@
+from UM.Scene.SceneNodeDecorator import SceneNodeDecorator
+
+##  A decorator that stores the amount an object has been moved below the platform.
+class ZOffsetDecorator(SceneNodeDecorator):
+    def __init__(self):
+        self._z_offset = 0
+
+    def setZOffset(self, offset):
+        print("setZOffset", offset)
+        self._z_offset = offset
+
+    def getZOffset(self):
+        return self._z_offset