- When composing the temporary virtual path for a new file, don't call

- When composing the temporary virtual path for a new file, don't call
  clean_path(). It is unnecessary and it also caused a problem if this path
  would begin with /../, because on UNIX systems, canonpath() removes /../ at
  the beginning of a path. So if a user wanted to create the file /../file.ext
  (which he wasn't allowed to), he created /file.ext.
- file_name() and upper_path() now remove multiple trailing slashes
- Improved configuration file parser:
  - Allow configuration options with empty values
  - If a option is defined twice, the line number is shown in the error message
- Static values are now surrounded by single quotes. Maybe it helps to increase
  the speed of Dev-Editor, because Perl doesn't have to try to interpolate
  variables in the values.