kinfocenter about-distro changes in 5.15.2+

[Harald Sitter]

Hello friends!

Plasma 5.15.2 will bring a number of improvements to the about-distro
KCM to allow you to rely more on os-release information and not so
much on the KCM's kcm-about-distrorc config.

- os-release's LOGO will be used by default (unless overridden by config)
- As a by-product the logo may be specified as an icon name for
resolution through the theme
- os-release's VARIANT will be used by default (unless overridden by config)
- The new config key UseOSReleaseVersion=true allows you to switch the
displayed version from VERSION_ID to VERSION. When using this please
be mindful of just how much text you have in the VERSION. It may make
the KCM look weird, in which case sticking to the default VERSION_ID
is advisable.

I should also note that in absence of the os-release values the
previous fallbacks continue to apply as usual. And to make to
perfectly clear: the rc continues to override whatever is set in
os-release, no changes there.

If you don't want to change anything you don't have to, but I hope
that these behavior adjustments will allow some of you to no longer
need the kcm-about-distrorc at all and fully rely on os-release.

HS