gutf8: Add a g_utf8_truncate_middle() function

Adds a helper to truncate UTF8 strings in the middle, allowing to make
them fit certain size constraints.

This function is modeled after similar functionality that has existed
since 2008 in nautilus and in eel before that.

docs/reference/glib/glib-sections.txt.in

@@ -3131,6 +3131,7 @@ g_utf8_strchr
 g_utf8_strrchr
 g_utf8_strreverse
 g_utf8_substring
+g_utf8_truncate_middle
 g_utf8_validate
 g_utf8_validate_len
 g_utf8_make_valid