chore: Build Windows ICO using ImageMagick

[jstarpl]

This PR adds a step to use ImageMagick to build the Windows ICO file.

[jpochyla]

Uuu, thanks a lot @jstarpl! Unfortunately it seems the Windows build is failing on the ICO step (with magick not found).

[jstarpl]

I'm gonna look into it some more and try to figure out why it can't find ImageMagick.

[jstarpl]

I think the problem was that the "desktop" way of setting the PATH across steps is no longer supported. Supposedly, this is how it should be done: https://stackoverflow.com/questions/60169752/how-to-update-the-path-in-a-github-action-workflow-file-for-a-windows-latest-hos . I tried to apply that solution, let's see if it works now.

[jstarpl]

Learning PowerShell is fun! /sarcasm. I think the correct string should now be written to the PATH GitHub variable.

[jstarpl]

Learning by doing. Hopefully now it's correct.

[jstarpl]

Okay, the PATH works now, I just missed a line creating a 32-bit 16x16 version.

[jstarpl]

Finally! The Windows build seems to work fine.

[jpochyla]

Thanks a lot @jstarpl! Merging <3