Add launchable tag to appdata

[City-busz]

This is needed for appstream-generator to find the associated desktop file.

See also:

• https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-launchable
• https://github.com/ximion/appstream-generator/issues/88

[davidmhewitt]

Thanks, I manually merged this here: https://github.com/davidmhewitt/torrential/commit/37500efbe02871b2f549367eaba45f695ea9d88f

Sorry it took so long to review and merge