BREAKING CHANGE (?): renaming the role

[PeterMosmans]

Currently the role's name customize-gnome does not adhere to Ansible's naming guidelines:

Role names are limited to lowercase word characters (i.e., a-z, 0-9) and ‘_’. No special characters are allowed, including ‘.’, ‘-‘, and space. During import, any ‘.’ and ‘-‘ characters contained in the repository name or role_name will be replaced with ‘_’.

To do: enforce the role's "correct" name to customize_gnome. Unsure whether this will affect existing installations. It shouldn't, but...