Added files for proper integration with the KDE sddm configuration module

I have added the files necessary to be able to use the kcm-sddm module. 

These include:
   metadata.desktop - where I didn't know which licence is the theme released under and left the default one (the one of the themes included with sddm) of CC-BY-SA
   theme.conf - setting the default background for the theme
   screenshot.png - a screenshot obtained with qmlviewer