Change gcode flavor labels to make it obvious which firmwares they are targetting.

Rename "RepRap (Marlin/Sprinter)" to "Marlin".
Rename "RepRap (Volumetric)" to "Marlin (Volumetric)".
Rename "RepRap (RepRap)" to "RepRap".

resources/definitions/fdmprinter.def.json

@@ -303,9 +303,9 @@
                     "type": "enum",
                     "options":
                     {
-                        "RepRap (Marlin/Sprinter)": "RepRap (Marlin/Sprinter)",
-                        "RepRap (RepRap)": "RepRap (RepRap)",
-                        "RepRap (Volumatric)": "RepRap (Volumetric)",
+                        "RepRap (Marlin/Sprinter)": "Marlin",
+                        "RepRap (Volumatric)": "Marlin (Volumetric)",
+                        "RepRap (RepRap)": "RepRap",
                         "UltiGCode": "Ultimaker 2",
                         "Griffin": "Griffin",
                         "Makerbot": "Makerbot",