Fix table header icon positioning and size

[polter-rnd]

This is basically a port from Fusion style.

Fixes #187

Problem occurs on specific monitor configuration:

• Wayland
• 2 monitors, one has fractional scaling
• Pixmap in QTableHeaderView drawn DPI-aware

It's difficult to add such case to demo/showcase app, since the icon has to be drawn by custom model. However it can be easily checked in KeePassXC app.

Before:

After: