Make serialisation version static

It needs to be accessed by GCodeReader to remain consistent.

Contributes to issue CURA-34.

plugins/GCodeWriter/GCodeWriter.py

@@ -9,6 +9,12 @@ import re #For escaping characters in the settings.
 
 
 class GCodeWriter(MeshWriter):
+    ##  Serialisation version ID of the settings that get serialised in g-code.
+    #
+    #   If the format of settings is modified in any way that breaks backwards
+    #   compatibility, this version number must be incremented.
+    settings_version_id = 1
+    
     def __init__(self):
         super().__init__()
 
@@ -36,8 +42,8 @@ class GCodeWriter(MeshWriter):
     #   \param profile The profile to serialise.
     #   \return A serialised string of the profile.
     def _serialiseProfile(self, profile):
-        version = 1 #IF YOU CHANGE THIS FUNCTION IN A WAY THAT BREAKS REVERSE COMPATIBILITY, INCREMENT THIS VERSION NUMBER!
-        prefix = ";SETTING_" + str(version) + " " #The prefix to put before each line.
+        #IF YOU CHANGE THIS FUNCTION IN A WAY THAT BREAKS REVERSE COMPATIBILITY, INCREMENT settings_version_id!
+        prefix = ";SETTING_" + str(self.settings_version_id) + " " #The prefix to put before each line.
         
         serialised = profile.serialise()