GPT support

[Khitiara]

Mostly making this for tracking.

• protective MBR partition type 0xEE (see also Add support for all MBR partition types by name and number. #8)
• GPT table writing
• partition type by GUID

[ikskuh]

Thank you! Feel free to tackle it yourself :) might be faster than me.

I merged the refactor, so changes and testing should be fairly easy now

[bnuuydev]

I'm almost finished with implementing this. Should be a week or two at the most to have a PR open :)