It would be very nice to have a function that could
be used in situations like gtk_dialog_new_with_markup()
that automatically escaped all string and character
arguments with g_markup_escape_text()

The problem here is implementing it. Problems with
doing the printf first:

 * Hard to implement - we need to be able to do something
   with only the parts of the string that are printf
   output.

Problems with parsing the markup first, then doing
the printf.

 * What does Should %<b>s</b> work as a string format?
   What is the right result?
 * Should #x2f;s work as a string format?
 * Hard to implement; how do we know how to adjust
   the attrlist for the effects of the expansion.

A trick we can use to implement this described as follows:

Say, we have 

 printf ("Jane %s the %s\N", verb, noun);

If we compare the results of:

 printf ("%sX%sX", verb, noun);
 printf ("%sX%sY", verb, noun);

And we get:

 "ateXcakeX"
 "ateYcakeY"

then by finding the next position at which the two strings
vary we can determine that the two formatted segments
were "ate" and "cake". 

The attached patch implements this, using a heavily stripped
down form of gnulib's printf-parse.c to find the conversion
specifiers. It's not especially efficient way of doing
things but should be quite portable.

Perhaps g_markup_escaped_printf() / g_markup_escaped_printfv()
for names for the functions.

The patch could use some more tests and some comments, and
needs documentation.