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