CrossPlatform Path fix: remove ":" in path names on Windows

In the logs of my dedicated server I found some error messages, that Widelands was unable to write a saved game file on the client side. It seemed that this error was based upon the ":" in the saved game name - valid on unix filesystems, but invalid on Windows.

Because of that, the function that fixes paths send via network ( "\" vs. "/" until now) should replace the ":" as well on Windows.