Better AppDir support

Better core App Dir support

Currently ROX-Filer uses the function rox_spawn() to start other programs. However it does not directly support App Dirs, each caller to rox_spawn() must do that itself and not all do. This means only partial support for our own format!
We should re-write rox_spawn() to directly support App Dirs itself, removing that burden from its callers. The various libraries should also support it.