Allow larger icon sizes

What is the reason to restrict the sizes to 16, 22 or 24 ?

It doesn't seem to be large enough for high DPI displays (in my case 2256 x 1504), so I built Hardcode-Tray locally to allow larger values for the `--size` argument.
32 was aesthetically the sweet spot for me, but I also played with even larger values and nothing went really bad.
Would you consider giving options for larger sizes ?

Start at the command-line handling for the `--size` argument and inspect where the current 16, 22, and 24 values are restricted. Done means larger values, including 32, are accepted and used appropriately for high-DPI tray icons; the payload names no specific files or tests.

Written by the indexing model from the issue text.