Set a fallback icon for the added MimeTypes

[Pacho Ramos]

Due to https://gitlab.gnome.org/GNOME/nautilus/-/issues/2796 , a ugly fallback icon will be used. Appending

will let a better icon to be displayed if no dedicated icons per mimetype are provided

This can be added with:

sed -i -e '/<_comment/a\\t\t<generic-icon name="image-x-generic"/>' src/chemical-mime-database.xml.in
sed -i -e 's:acronym|alias|comment|:acronym|alias|comment|generic-icon|:' xsl/cmd_freedesktop_org.xsl

Thanks a lot