Make gggenomes ready for CRAN submission

[iimog]

I'm trying to fix all the Errors, Warnings and Notes of R CMD check. I already fixed a couple. Still getting errors in the examples. Status is:

Status: 1 ERROR, 7 WARNINGs, 4 NOTEs

The 1 Error is misleading, because only fixing one turns up the next one :sweat_smile:

I'll continue fixing stuff and add it to this pr. But all changes I made so far should be safe to merge anytime, to avoid conflicts in the future.