[cosmetics] Pretty-align some code
[command] Introduce 'Remove unused @prefix directives' command

Checks which prefixes are declared but unused and removes them. The
user will be informed about any deletions.
[command] Introduce 'Convert to format' menu command

Allows easy conversion between common RDF serialization formats.
Depends on Raptor.
Rearrange menu items
[help] Update README to reflect recent additions