Improve file dialogs
- Fixes #314
- Adds file extensions to sound and image browsing
- Some options no longer default to our config dir
- Always add a shortcut to our config dir
- Remove broken 'last_dir' functionality, gtk already knows recent
Use mimetypes for audio and images
Of course mimetype doesn't work on Windows..
Merge pull request #733 from hexchat/filedialog

Improve file dialogs