If it's at all possible to use `const std::string&` when passing
`std::string`s around, then we use it. This is to limit the amount of
memory usage as a result of the frequent use of `std::string`s, so the
game no longer unnecessarily copies strings when it doesn't need to.
This is a pretty hefty commit! But essentially, I made a new editorclass
object, and moved all functions and variables that only get used in the
in-game level editor to that class. This cleanly demarcates which things
are in the editor and which things are just general custom level stuff.
Then I fixed up all the callers. I also fixed up some NO_CUSTOM_LEVELS
and NO_EDITOR ifdefs, too, in several places.
