* Workaround for Qt 5 bug #40681.

* Workaround for Qt 5 bug #40681 by monitoring QScreen::destroyed() and re-create affected panels manually.
* This closes lxde/lxde-qt bug #204, #205, and #206 at the same time.
* Store the result of QX11Info::display() and avoid repeated calls to the methods.
(When called during the primary screen being destroyed, QX11Info::display() crashes.)