Create default config file.
Remove key tag from config file.
Load config file.
Prepare default config.
Set window size from config.
Remove fullscreen option from config.
Set external image viewer from config.
Build metadata fields from config.
Rearrange and activate the default config.
Allow a one-field-only config.
Find config file.
Add descriptive comments to the config file.
Read allowed file extensions from config file.
Update installation instructions.
Add configuration instructions.
Update TODO items in README file.
Refine the config file's descriptive comments.
Update the README's intro.