Remove leading and trailing whitespace when choosing filenames. Fixes is...

...sue #401.

In Windows this enabled users to create folders/files with leading/trailing space in the filename. These folders/files cannot be removed through the GUI nor qbt can delete them. Special cmd commands are needed for these files to be removed from the filesystem.

On a side-note: This check could happen inside fsutils::isValidFileSystemName() to universally prohibit such use in the future too. But it has one flaw. Normally the user won't understand why "*The file could not be renamed*" when the illegal character is leading/trailing whitespace. Furthermore, I think we can safely assume that whitespace should be removed automatically.