Optimize
- optimize: draw only the parts that changed (mem and swap don't update too often)
  -> probably not possible, as QStyleOptionGraphicsItem::exposedArea() contains what's visible (and usually the whole item is visible)

Ideas
- use oxygen colors, or the colors plasma or kcolorscheme provides
- use KColorUtils::shade(col, KColorUtils(Dark/Light)) instead of QColor::lighter()/darker()
  -> only if it's not slower
- tooltip: show frequency of the processors
- middle mouse button or in context menu: launch ksysguard