Field names inspired by KPluginMetaData keys.
bugReportUrl yet needs an entry for libkmahjongg in bugs.kde.org.
Also not yet sure how to display in UI,
Not yet sure how to display BugReportUrl in UI, but adding already so
3rd-party can consider to use it.
With copyright & license info added to the desktop file, the separate copyright files should no longer be needed.
I could not see a discussion about the copyright files from the commit which introduced them when desktop files were also introduced: https://phabricator.kde.org/R883:610889
The old .tileset files had no respective info at least.