Improve hidpi toolbar

As shown in attachment, the main toolbar icons rendering is sub-optimal when a scaled UI is used on a high dpi screen. This merge request adds setting of QPainter::SmoothPixmapTransform render hint in the paint functions of the toolbar buttons, producing better-looking results on 150 % and 200 % scaling, as shown in "After".

(Also shown in After #2 (closed), preliminary result when the svg's are rendered to bigger pixmaps, however since that requires much broader changes to code than the one-liners in this merge request to get to the first After, I'll work on that separately)

Edit: Progress slider before-after