Renames the Qt5 build

Qt4 and Qt5 build will coexist in many systems. They must have a
different name so they will not clash.
Qt5Xdg is the name for the Qt5 build. All the other installables are
installed into an qt5xdg dir.

Signed-off-by: Luís Pereira <luis.artur.pereira@gmail.com>
Adds pkg-config to Qt5Xdg

Signed-off-by: Luís Pereira <luis.artur.pereira@gmail.com>
Replace QDomNodeList::length() by QDomNodeList::size()

QDomNodeList::length() returns an uint in Qt4 and an int in Qt5.
But QDomNodeList::size() returns an int in both of them. Apart from the
return type size() and length() are equivalent.

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