This allows more consistency in the manner we include core headers,
where previously there would be a mix of absolute, relative and
include path-dependent includes.
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.
Rémi Verschelde
Michael Alexsander Silva Dias