Include paths are processed from left to right, so we use Prepend to
ensure that paths to bundled thirdparty files will have precedence over
system paths (e.g. `/usr/include` should have lowest priority).
Modules can register their own editor plugins (like GridMap does),
so no need to put module-specific classes in the `editor/` folder.
Also cleans up the previous SCons env pollution from the Recast
module, integrating its code into libmodules as other modules.
