Set fixed standard icon sizes for header toolbar icons

These are line-art so they should be using a fixed and standard icon
size rather than growing with the font size. This also fixed various
binding loop issues with setting implicit sizes from calculated sizes.
2 jobs for master in 14 minutes and 10 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #69694
linux kf5-qt5 qt5.12

00:14:09

86.0%
passed #69693
reuse

00:00:28