Tweak display of hidden items in "1 Player" menu

This merge request makes it so that if an option in the "1 Player" menu is unvailable in the current add-on, rather than being hid and leaving an empty space in the menu, it will be hid and then all above options will be lowered to close the gap.

This merge request also fixes Marathon Run being hid instead of shown as question marks if it's not currently unlocked.

It also changes sploadgame in m_menu.c to spstartgame, since it's used for the "Start Game" option.

Before/After comparisons, with vanilla SRB2 data first, and then an with add-on that disables NiGHTS Mode and Tutorial, but leaves Record Attack and Marathon Run enabled:

|Context|Before|After| |::|::|::| |Vanilla, brand-new||| |Vanilla, completed||| |Add-on, brand-new||| |Add-on, completed|||

TL;DR:
Improved display of hidden items in "1 Player" menu.
(Also made Marathon Run show as question marks when locked, but this doesn't apply to the changelog for v2.2.5, as Marathon Run didn't exist in v2.2.4.)