- Let's just have one header definining export- and visibility-
macros for Calamares. They are still selected based on the
export flags (*_PRO), just defined in one header instead of two.
- These methods are used for multi-page view-steps, which are rare.
For all the others, just drop the empty implementation and defer
to the base class.
- Document netinstall.conf a little,
- Add setting *required* which influences whether next is enabled or not
in case of missing or corrupt data,
- Enable *next* button only once some (any!) data is received.
This can be used to disallow stepping past the netinstall step when
there is no data (e.g. internet has failed between the welcome page
and the netinstall page).
