[about-distro] run logo lookup via osrelease

Summary:
new preference:

• kcm-about-distrorc
• os-release
• hardcoded start-here-kde

this removes the need to duplicate the logo information in the kcm rc if
a distro already has it encoded in the os-release's LOGO value

CHANGELOG: About System can now use os-release's LOGO configuration

Test Plan: logo can be set from os-release

Reviewers: apol

Reviewed By: apol

Subscribers: apol, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D19181