Revert commit 191fc15b9f to use closeEvent()
but without QEvent::ignore() which blocked logout on various Linux desktops,
and without redundant hide(). Previous fix with hideEvent() produced
regressions because hide() is used in various places, which produced closing
behavior.
Since qApp->setQuitOnLastWindowClosed(false) is used, and
Qt::WA_DeleteOnClose is *NOT* used, default closeEvent() handling is enough
for implementing close-to-systray feature.
Improves fix for #1485 and closes#3699
6b00779dde