High resolution screens are hitting limits in icons size

We are hitting the limits imposed by available icons sizes on ports with the high screen resolutions. In particular, Sony XZ3 and XZ2 Compact could use larger pixel_ratio than available when choosing icons. Thus, we would need sailfish-content-graphics-z{} with higher z values.

Opening it here as I have just got a new issue filed in the port repository regarding small icons.

Related TJC thread: https://together.jolla.com/question/220290/pixel_ratio-limitation-for-high-resolution-screens/