QIconLoader: don't re-evaluate container.size() all the time
QIconLoader: don't re-evaluate container.size() all the time

Experience shows that the compiler does not recognize the
size() call as a constant expression, so help it along.

Port of commit
https://qt.gitorious.org/qt/qtbase/commits/5335a7153549fcb66f357c04b3b94e3ab112dc4e

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