Add .desktop file and logo for appimage

The logo is actually extracted from the site, but since it's just for
the appimage (I don't even know where it shows it, tbh) it's okay for
now.

Progress towards #6475.

[ci skip]

fish.desktop

@@ -0,0 +1,12 @@
+[Desktop Entry]
+Encoding=UTF-8
+Version=1.0
+Type=Application
+Terminal=true
+# This file is for use in appimages, which rewrite the path.
+# Other uses would have to replace these with the correct path.
+Exec=fish
+Name=fish
+Icon=fish
+# "Shell" would be the proper category, but it's reserved so we can't use it.
+Categories=ConsoleOnly;System;