Only moves code. The tabs other than System were built by static functions
in GameSettingsScreen.cpp, in place of the old methods, to keep the previous
change's diff readable. Now each tab has its own file, with just the
includes it needs, and MainSettingsPages.h declares their builders.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
