Enhancement: Add Cura version to g-code

[ArizonaClark]

As a Software Engineer for over 30 years (mostly custom compilers), I strongly suggest that you add a comment of "; Sliced by Cura {cura_version}" to the default start.gcode -- this can make it a lot easier for both the users AND the developers in tracking down bugs.

[Ghostkeeper]

I don't expect there to be such features added to Legacy Cura. Development for this version has stopped. We continue our work in Ultimaker's branch: https://github.com/Ultimaker/Cura

New versions put this message at the top of the file:

;Generated with Cura_SteamEngine 2.x.x