Windows supports using slashes except of backslashes in all but a few documented cases. And with the current gcl tofiledir doesn't allow for backslashes anymore and wxMaxima cannot open files with path names containing backslashes => why not generally replace backslashes by slashes?
Andrej Vodopivec is right: 1.) This patch is needed for windows only and 2.) there is a dedicated function that does do exactly what I want from it => revised the patch.
Merge pull request #364 from gunterkoenigsmann/BackslashAsSlash

PWDs containing backslashes still fail to be set on windows.