[about-distro] let distributions choose VERSION_ID or VERSION

Summary:
by default we display VERSION_ID because it is the pure numeric
representation while VERSION may contain all sorts of clutter information
that overloads the KCM and possibly makes the KCM look ugly.
but, VERSION is technically meant for displaying to the user, so there
is a reason why a distribution may choose to display that instead of the
ID.

to give them control over this there's now a bool config entry
UseOSReleaseVersion which defaults to false, when set to true the KCM
will use the VERSION instead of VERSION_ID.

Test Plan:
- VERSION and VERSION_ID are different
- config value not set
- kcm uses VERSION_ID
- config value set to true
- kcm uses VERSION

Reviewers: apol

Reviewed By: apol

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D19226