Add basic UFP writer plug-in

This already implements most of it. Just the thumbnail must still be gotten and put inside the container.

Contributes to issue CURA-4872.

plugins/UFPWriter/UFPWriter.py

@@ -0,0 +1,21 @@
+#Copyright (c) 2018 Ultimaker B.V.
+#Cura is released under the terms of the LGPLv3 or higher.
+
+import charon
+
+from UM.Mesh.MeshWriter import MeshWriter #The writer we need to implement.
+from UM.PluginRegistry import PluginRegistry #To get the g-code writer.
+
+class UFPWriter(MeshWriter):
+    def write(self, stream, nodes, mode = MeshWriter.OutputMode.BinaryMode):
+        archive = charon.VirtualFile()
+        archive.open(stream, charon.OpenMode.WriteOnly)
+
+        #Store the g-code from the scene.
+        archive.addContentType(extension = "gcode", mime_type = "text/x-gcode")
+        gcode = archive.getStream("/3D/model.gcode")
+        PluginRegistry.getInstance().getPluginObject("GCodeWriter").write(gcode, None)
+        archive.addRelation(target = "/3D/model.gcode", file_type = "http://schemas.ultimaker.org/package/2018/relationships/gcode")
+
+        #Store the thumbnail.
+        #TODO