Including a header file inside another header file means a bunch of
files are going to be unnecessarily recompiled whenever that inner
header file is changed. So I minimized the amount of header files
included in a header file, and only included the ones that were
necessary (system includes don't count, I'm only talking about includes
from within this project). Then the includes are only in the .cpp files
themselves.
This also minimizes problems such as a NO_CUSTOM_LEVELS build failing
because some file depended on an include that got included in editor.h,
which is another benefit of removing unnecessary includes from header
files.
I think it's a bit silly to always include the Steam and GOG APIs
whenever we build VVVVVV, since the only time they'll ever be used is in
a live build and not a dev build.
So now Steam and GOG are disabled by default. If you want them, you'll
need to add -DSTEAM=ON or -DGOG=ON respectively at CMake time. They're
also both automatically enabled for release builds.
