Implementing set_window_icon on Linux

[GreenXenith]

Opening this because I don't currently understand how to execute the theoretical steps.

In theory, this is achieved by loading the image, converting to RGBA, turning it into a pixmap, and setting it as the _NET_WM_ICON property using XChangeProperty.
Someone with more understanding of C++ and images should be able to implement this with relative ease.

It is worth noting xseticon, which uses libgd for color conversions and loading, but I am not certain if this is helpful.