Ticket #3184: minor skin cleanups.

  * Consistent indentation: All skins use now 4 spaces (the default of
mcedit). This makes it easier to copy-paste sections from one skin to
another, while maintaining consistency.
  * Spaces around the '=' sign. The version with spaces is more
readable, especially around double line characters.
  * Make the order of the sections consistent across all skins.
  * In [menu], swap the order of "menusel" and "menuhot" to make the
order consistent with the [dialog] and [error] sections.
  * Add the missing "helptitle" to sand256 skin. This is the only
functional change.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>