Rename the desktop and AppStream data files to conform to the AppStream
1.0 specification; these now have names based on "au.org.zap.trader".
Add a caption to the AppStream screenshot; update all translations to
suit. Rename msgfmt-appdata to msgfmt-metainfo to reflect these changes.
The reStructuredText (ReST) format is still quite readable as raw text,
and is much more capable than the myriad all-slightly-incompatible
variants of Markdown.
