[about-distro] run absolute paths through qicon intead of qpixmap

Summary:
qicon::fromtheme internally skips resolution of absolute paths and instead
simply opens it by path. so, simplify the code by only calling fromTheme.

this has two additional advantages:
- the icon size is now always capped at 128x128 (smaller icons are smaller
   but that beats upscaling) preventing problems where a distribution might
   use too large a resolution so the kcm looks wonky on scaling factor 1
- the LogoPath config may now also be an icon name for resolution through
   the icon theme instead of an absolute path

Test Plan: - huge logos are scaled into shape

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: davidedmundson, ngraham, broulik, cfeck, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D19033