Do not allow notifications popups to overlap the panel

There is a problem in QDesktopWidget::availableGeometry(). To fix the problem, [LXQt::xfitMan::availablegeometry()](https://github.com/lxde/liblxqt/blob/master/lxqtxfitman.cpp#L840) is used.

Qt bug: https://bugreports.qt-project.org/browse/qtbug-18380