[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