[Request] Add lookup code for icon (from dekstop file) in window mode.

I was about to make a new request, but I guess this one is close enough.

It would be really useful if we have unified icons between drun and window in the same way that a dock, for example plank, would do. This way I am not confused when I see a different icon from what I launched. This will greatly increase the intuitiveness of window modi.

Those docks don't use NET_WM_ICON, instead they somehow pull icon names from .desktop files even if software doesn't get launched from a .desktop file. I don't know how they do it, maybe check plank for that.

Having the wrong icon is like typing on a new keyboard layout, it totally kills the flow.

Originally posted by @oredaze in #1935