Broken HighDPI scaling on Plasma

[nicolasfella]

UI elements in Gwenview's annotation tool (which uses kImageAnnotator) are too large when using HighDPI scaling. See https://bugs.kde.org/show_bug.cgi?id=479215

It worked fine until #302, which is conceptually wrong. Qt already does proper scaling out of the box. Manually multiplying a scale factor onto things is wrong in most cases and causes things to be scaled twice.