Implement version upgrade 2.7 to 3.0

The theme 'cura' was renamed to 'cura-light', so that needs to be updated in the user's preferences.

Contributes to issue CURA-4148.

plugins/VersionUpgrade/VersionUpgrade27to30/__init__.py

@@ -0,0 +1,23 @@
+# Copyright (c) 2017 Ultimaker B.V.
+# Cura is released under the terms of the AGPLv3 or higher.
+
+from . import VersionUpgrade27to30
+
+upgrade = VersionUpgrade27to30.VersionUpgrade27to30()
+
+def getMetaData():
+    return {
+        "version_upgrade": {
+            # From                    To                       Upgrade function
+            ("preferences", 4000002): ("preferences", 5000002, upgrade.upgradePreferences),
+        },
+        "sources": {
+            "preferences": {
+                "get_version": upgrade.getCfgVersion,
+                "location": {"."}
+            },
+        }
+    }
+
+def register(app):
+    return { "version_upgrade": upgrade }