app: add a comment and some newlines before the debug data section.

The markdown triple-quote (```) has to be on its own at the start of a
newline. Schumaml was telling me that too many reporters would paste
just after some text, which would therefore break the markdown syntax.

Instead let's add 3 newlines before the triple-quote, so that even
people who would not hit the "Copy Bug Information" button (but instead
select and copy) have a hint that these newlines are made on purpose.
Also add a comment (which is discarded by Gitlab) to make this even more
obvious.

Then even when pasting just after some text on the same line, the
triple-quote will end up on its own line.

app/widgets/gimpcriticaldialog.c

@@ -259,7 +259,9 @@ gimp_critical_dialog_constructed (GObject *object)
 
   buffer = gtk_text_buffer_new (NULL);
   version = gimp_version (TRUE, FALSE);
-  text = g_strdup_printf ("```\n%s\n```", version);
+  text = g_strdup_printf ("<!-- %s -->\n\n\n```\n%s\n```",
+                          _("Copy-paste this whole debug data to report to developers"),
+                          version);
   gtk_text_buffer_set_text (buffer, text, -1);
   g_free (version);
   g_free (text);