qt: Add Distinguished Name parser from libkleo

* lang/qt/src/dn.cpp (DN, DN::Attribute): New public API.
* lang/qt/src/dn.h: New.
* lang/qt/src/Makefile.am: Update accordingly.

This is a general useful API to work with X509 distinguished
names and is useful when doing CMS with GpgME.

Adding this class from libkleo allows kmails messagelib to
avoid any KDE UI Frameworks and so can be used more versatile
in the future.

This class can be combined with libkleos DNAttributeMapper
to have the same bevavior as before in libkleo when using
DN::prettyDN calls can be converted from:

Kleo::DN(uid).prettyDN();

to:

QGpgME::DN dn(uid);
uid.setAttributeOrder(
    Kleo::DNAttributeMapper::instance()->attributeOrder());
dn.prettyDN();