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/VersionUpgrade27to30.py

@@ -0,0 +1,56 @@
+# Copyright (c) 2017 Ultimaker B.V.
+# Cura is released under the terms of the AGPLv3 or higher.
+
+import configparser #To parse preference files.
+import io #To serialise the preference files afterwards.
+
+from UM.VersionUpgrade import VersionUpgrade #We're inheriting from this.
+
+_renamed_themes = {
+    "cura": "cura-light"
+}
+
+class VersionUpgrade27to30(VersionUpgrade):
+    ##  Gets the version number from a CFG file in Uranium's 2.7 format.
+    #
+    #   Since the format may change, this is implemented for the 2.7 format only
+    #   and needs to be included in the version upgrade system rather than
+    #   globally in Uranium.
+    #
+    #   \param serialised The serialised form of a CFG file.
+    #   \return The version number stored in the CFG file.
+    #   \raises ValueError The format of the version number in the file is
+    #   incorrect.
+    #   \raises KeyError The format of the file is incorrect.
+    def getCfgVersion(self, serialised):
+        parser = configparser.ConfigParser(interpolation = None)
+        parser.read_string(serialised)
+        format_version = int(parser.get("general", "version")) #Explicitly give an exception when this fails. That means that the file format is not recognised.
+        setting_version = int(parser.get("metadata", "setting_version", fallback = 0))
+        return format_version * 1000000 + setting_version
+
+    ##  Upgrades a preferences file from version 2.7 to 3.0.
+    #
+    #   \param serialised The serialised form of a preferences file.
+    #   \param filename The name of the file to upgrade.
+    def upgradePreferences(self, serialised, filename):
+        parser = configparser.ConfigParser(interpolation=None)
+        parser.read_string(serialised)
+
+        # Update version numbers
+        if "general" not in parser:
+            parser["general"] = {}
+        parser["general"]["version"] = "5"
+        if "metadata" not in parser:
+            parser["metadata"] = {}
+        parser["metadata"]["setting_version"] = "2"
+
+        #Renamed themes.
+        if "theme" in parser["general"]:
+            if parser["general"]["theme"] in _renamed_themes:
+                parser["general"]["theme"] = _renamed_themes[parser["general"]["theme"]]
+
+        # Re-serialise the file.
+        output = io.StringIO()
+        parser.write(output)
+        return [filename], [output.getvalue()]