Added Support Eraser Plugin

This is the first draft of the Support Eraser tool. The plugin creates a cube mesh which has the `anti_overhang_mesh` decorator causing it to block the creation of support material for overhangs within its volume. This distinction is necessary to avoid confusing this behavior from actually erasing a _portion_ of a support structure.

Some (non-necessary) improvements could be made such as:

- Better graphical style
- Discussion of whether the cube should be able to pass through the build plate or not
- Possible improvements to the tool's icon
- Placing the cube at a cursor click location

plugins/SupportEraser/SupportEraser.py

@@ -0,0 +1,68 @@
+from UM.Tool import Tool
+from PyQt5.QtCore import Qt, QUrl
+from UM.Application import Application
+from UM.Event import Event
+from UM.Mesh.MeshBuilder import MeshBuilder
+from UM.Operations.AddSceneNodeOperation import AddSceneNodeOperation
+from UM.Settings.SettingInstance import SettingInstance
+from cura.Scene.CuraSceneNode import CuraSceneNode
+from cura.Scene.SliceableObjectDecorator import SliceableObjectDecorator
+from cura.Scene.BuildPlateDecorator import BuildPlateDecorator
+from cura.Settings.SettingOverrideDecorator import SettingOverrideDecorator
+
+import os
+import os.path
+
+class SupportEraser(Tool):
+    def __init__(self):
+        super().__init__()
+        self._shortcut_key = Qt.Key_G
+        self._controller = Application.getInstance().getController()
+
+    def event(self, event):
+        super().event(event)
+
+        if event.type == Event.ToolActivateEvent:
+
+            # Load the remover mesh:
+            self._createEraserMesh()
+
+            # After we load the mesh, deactivate the tool again:
+            self.getController().setActiveTool(None)
+
+    def _createEraserMesh(self):
+        # Selection.clear()
+
+        node = CuraSceneNode()
+
+        node.setName("Eraser")
+        node.setSelectable(True)
+        mesh = MeshBuilder()
+        mesh.addCube(10,10,10)
+        node.setMeshData(mesh.build())
+
+        active_build_plate = Application.getInstance().getBuildPlateModel().activeBuildPlate
+
+        node.addDecorator(SettingOverrideDecorator())
+        node.addDecorator(BuildPlateDecorator(active_build_plate))
+        node.addDecorator(SliceableObjectDecorator())
+
+        stack = node.callDecoration("getStack") #Don't try to get the active extruder since it may be None anyway.
+        if not stack:
+            node.addDecorator(SettingOverrideDecorator())
+            stack = node.callDecoration("getStack")
+
+        print(stack)
+        settings = stack.getTop()
+
+        if not (settings.getInstance("anti_overhang_mesh") and settings.getProperty("anti_overhang_mesh", "value")):
+            definition = stack.getSettingDefinition("anti_overhang_mesh")
+            new_instance = SettingInstance(definition, settings)
+            new_instance.setProperty("value", True)
+            new_instance.resetState()  # Ensure that the state is not seen as a user state.
+            settings.addInstance(new_instance)
+
+        scene = self._controller.getScene()
+        op = AddSceneNodeOperation(node, scene.getRoot())
+        op.push()
+        Application.getInstance().getController().getScene().sceneChanged.emit(node)