Updates the headers naming.

WARNING: The people still using #include <qtxdg/xdgdesktopfile.h> or
#include <qtxdg5/xdgdesktopfile.h> mustupdate their code. It will not work
anymore.

People using #include <XdgDesktopFile> are Ok. It works just fine. That's
not the preferred way anymore but will continue working for some time.
The preferred way to include the headers is:
#include <QtXdg/XdgDesktopFile>
It prevents name clashes and works (as #include <XdgDesktopFile>) for Qt4
and Qt5.

Signed-off-by: Luís Pereira <luis.artur.pereira@gmail.com>