Command line auto-completion enhancements

Command line auto-completion enhancements

- Limit completions to a list of file extensions known to Geeqie
- Limit completions to a specific extension where possible e.g. gqv
- Change option +t to -T and +w to -w. The plus character affected the
output of _parse_help
- Additional warning to user for invalid parameters - unknown option or
valid remote option used without a --remote command
- New remote.cc function for above feature
- Update man page and documentation