add gpu entry to about system

Summary:
simply queries gl_renderer from gl

also refactor string processing for (R) and (TM) replacement so
it can be shared for cpu and gpu entries

I do wonder if we need to blacklist software gl somehow though

BUG: 355053
FIXED-IN: 5.19.0

Test Plan:
shows something of value on amd and intel

depends on D26667

Reviewers: davidedmundson, ngraham

Reviewed By: davidedmundson, ngraham

Subscribers: ngraham, broulik, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D26709