Introduce an ExtruderStack class

This will allow us to codify some of the assumptions made about
extruders.

Contributes to CURA-3497

cura/Settings/ExtruderStack.py

@@ -0,0 +1,19 @@
+# Copyright (c) 2017 Ultimaker B.V.
+# Cura is released under the terms of the AGPLv3 or higher.
+
+from UM.MimeTypeDatabase import MimeType, MimeTypeDatabase
+from UM.Settings.ContainerStack import ContainerStack
+from UM.Settings.ContainerRegistry import ContainerRegistry
+
+class ExtruderStack(ContainerStack):
+    def __init__(self, container_id, *args, **kwargs):
+        super().__init__(container_id, *args, **kwargs)
+
+extruder_stack_mime = MimeType(
+    name = "application/x-cura-extruderstack",
+    comment = "Cura Extruder Stack",
+    suffixes = [ "extruder.cfg" ]
+)
+
+MimeTypeDatabase.addMimeType(extruder_stack_mime)
+ContainerRegistry.addContainerTypeByName(ExtruderStack, "extruder_stack", extruder_stack_mime.name)