OS Name and Version as Theme Properties

[Videowelder]

Working on a theme that displays the detected OS as a graphical design element, much like some distros did with lightdm in the past.
Figured these properties would be of use on the official project for other theme designers.
These use built-in Qt stuff, so wide support and no maintenance required.

[Vogtinator]

I think this can be implemented solely in the greeter, no need to touch the daemon at all

[Vogtinator]

IMO QSysInfo::prettyProductName() is the most important one and should be available as well.

Then we have three new properties, at which point it might be a good idea to use an object with properties instead, allowing for use from QML like this:

console.log("OS: " + sddm.os.prettyName + " Version: " + sddm.os.version)