New grubcfg module, ported from Thus.

With this module we configure the /etc/default/grub file and add the Distribution name, optionally resume and optionally plymouth to the configuration.
Configurability is good, but I rather expect to expose this through libcalamares once we get a proper branding system for all of Calamares (timeframe: 3-4 weeks from now). Ok to merge with this setting, but ideally we'll be removing it in the future.