[rkward] rkward: Move AppData file to desktop, install it, minor tweaks

[meik michalke]

hi yuri,

thanks for enhancing the AppData file! the screenshot section might need to be
replaced though, as both the comments in the template and the guidelines tell
us to use
https://phabricator.kde.org/source/websites-product-screenshots/
as the central screenshot location:
https://community.kde.org/Guidelines_and_HOWTOs/AppStream
https://phabricator.kde.org/source/websites-product-screenshots/browse/
master/README.md


viele grüße :: m.eik

+ <screenshots>
<screenshot type="default">
+ <caption>Working with data in RKward</caption>
<image>
- see
https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-> screenshots - Central hosting for uploading at
https://phabricator.kde.org/source/websites-product-screenshots/ -
Central URL to use here https://cdn.kde.org/screenshots/... +
Loading Image...
</image>
</screenshot>
- </screenshots> -->
+ </screenshots>