Add a PlatformPhysics class to handle some details about keeping objects on the printer platform

PlatformPhysics.py

@@ -0,0 +1,39 @@
+from UM.Scene.SceneNode import SceneNode
+from UM.Scene.Iterator.BreadthFirstIterator import BreadthFirstIterator
+from UM.Operations.TranslateOperation import TranslateOperation
+from UM.Math.Float import Float
+from UM.Math.Vector import Vector
+from UM.Application import Application
+
+import threading
+
+class PlatformPhysics:
+    def __init__(self, controller):
+        super().__init__()
+        self._controller = controller
+        self._controller.activeToolChanged.connect(self._onActiveToolChanged)
+        self._active_tool = None
+        self._operation = False
+        self._onActiveToolChanged()
+
+    def _onActiveToolChanged(self):
+        if self._active_tool:
+            self._active_tool.endOperation.disconnect(self._onEndOperation)
+
+        self._active_tool = self._controller.getActiveTool()
+
+        if self._active_tool:
+            self._active_tool.endOperation.connect(self._onEndOperation)
+
+    def _onEndOperation(self):
+        root = self._controller.getScene().getRoot()
+        for node in BreadthFirstIterator(root):
+            if type(node) is not SceneNode:
+                continue
+
+            bbox = node.getBoundingBox()
+            if not Float.fuzzyCompare(bbox.bottom, 0.0):
+                self._signal_source = node
+                op = TranslateOperation(node, Vector(0, -bbox.bottom, 0))
+                Application.getInstance().getOperationStack().push(op)
+                self._signal_source = None