Based on work already existing in glib and dbus, two new functions to validate
and reformat utf-8 strings are added:

 * utf8_validate: returns true if the `char` sequence is valid utf-8.

 * utf8_make_valid: transforms an invalid utf-8 sequence into a valid one,
   by replacing each bad byte by U+FFFD REPLACEMENT CHARACTER. The same
   behaviour is implemented in other GNOME applications.

These two new functions are used in sip_utils::parseDisplayName().

Change-Id: Ie9a6c35b4df7883637374c325b05b6cf875f6b12
Signed-off-by: Pascal Potvin <pascal.potvin@extenway.com>