Add more Godot MIME icons

This PR builds up from !39 (merged) by improving the existing one and adding icons for all the rest of Godot's file types:

I've tested the icons on other DEs using the mimetypes from a flatpak Godot install and everything works fine. Ironically it doesn't appear to work on Plasma itself. Here it shows XFCE's Thunar together with Dolphin:

Already reported this in the bug tracker, but sadly it seems to have reached a halt.