Give Python modules access to the branding information.

This set of commits fixes the issue that Python modules need their own branding settings. It exports the branding settings from libcalamaresui to the GlobalStorage, where Python modules can easily read them. In addition, it adds a shortProductName setting to branding.desc, because it's the one short version that was missing and because that's what's really wanted in boot loader configurations. It also updates the `GlobalStorage.md` documentation and ports the `bootloader` and `grubcfg` modules to the new setting (also ensuring that special characters in it will not cause problems).