Introduce a GlobalStack class

This will allow us to codify some of the assumptions made about the
global stack

Contributes to CURA-3497

cura/Settings/GlobalStack.py

@@ -0,0 +1,22 @@
+# 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 CannotSetNextStackError(Exception):
+    pass
+
+class GlobalStack(ContainerStack):
+    def __init__(self, container_id, *args, **kwargs):
+        super().__init__(container_id, *args, **kwargs)
+
+global_stack_mime = MimeType(
+    name = "application/x-cura-globalstack",
+    comment = "Cura Global Stack",
+    suffixes = [ "global.cfg" ]
+)
+
+MimeTypeDatabase.addMimeType(global_stack_mime)
+ContainerRegistry.addContainerTypeByName(GlobalStack, "global_stack", global_stack_mime.name)