Fallback icon set now has general MIME icons support

Specifically, it supports all the XDG x-generic icons.

internal/util/util.go

@@ -0,0 +1,10 @@
+package util
+
+import "git.tebibyte.media/tomo/tomo/data"
+
+func GeneralizeXDGIconMimeType (mime data.Mime) data.Mime {
+	// FIXME make this more accurate
+	// https://specifications.freedesktop.org/icon-naming-spec/icon-naming-spec-latest.html
+	mime.Subtype = "x-generic"
+	return mime
+}