Please describe the problem:
g_markup_escape_text does not properly escape characters not allowed in XML using entities.

Steps to reproduce:
1. Create markuptest.c with the following content:

#include <stdio.h>
#include <glib.h>

main() {
	char input[1];
	input[0] = 0x0f;
	
	gchar *out = g_markup_escape_text(input, 1);
	
	return printf("<tag>%s</tag>",out);
}

2. Compile & link it
3. Install libxml2 (if not done already)
4. Run:
./markuptest | xmllint -

Actual results:
-:1: parser error : PCDATA invalid Char value 15
<tag></tag>
     ^



Expected results:
Output of something like <tag>&#x0f;</tag>

Does this happen every time?
yes

Other information:
It's probably necessary to implement the list described in http://www.w3.org/TR/REC-xml/#NT-BaseChar into this function and escape all other characters.