Replace QDomNodeList::length() by QDomNodeList::size()
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 <95d1463b1ff9eef535e39fb0d755bc242cfcea61@gmail.com>

gilir/libqtxdg-qt5,luis-pereira/libqtxdg,DrTobe/libqtxdg,lxde/libqtxdg,luis-pereira/libqtxdg,lxde/libqtxdg,libqtxdg/libqtxdg,DrTobe/libqtxdg,DrTobe/libqtxdg,gilir/libqtxdg-qt5,libqtxdg/libqtxdg