Expand os-release spec with LOGO

[hellcp at opensuse.org]

Hi,

A few months ago a discussion happened about changing display logo for 
Gnome
depending on os-release file [1].

However spec [2] doesn't cover DEs wanting to add items to it, just 
OSes, which
will still not be great, if support would have to be extended to 
SUSE_LOGO,
DEBIAN_LOGO, UBUNTU_LOGO etc.

KDE does this by having that stuff specified in 
/etc/xdg/kcm-about-distrorc [3],
which is not great, because again, it's kind of info that easily could 
be in
os-release for every DE and app that might need it. Both DEs require 
information
about logo location, would be nice to provide it once, for all that 
might require
it in the future.

Suggestion I would give about it would be to allow for either full path 
or a
name to be used with xdg-icon standard.

[1] https://bugzilla.gnome.org/show_bug.cgi?id=695691
[2] https://www.freedesktop.org/software/systemd/man/os-release.html
[3] 
https://github.com/openSUSE/plasma-opensuse/blob/leap-15.x/config-files/etc/xdg/kcm-about-distrorc
LCP [Stasiek]
https://lcp.world