Fix focus changing when unminimising on X11
In f220e3b0 I made the keyboard focus move to the places and terminal panel whenever they are toggled visible. Unfortunately the QDockWidget::visibilityChanged() signal is also emitted (at least on X11) simply when the window containing that panel is minimized or restored. This commit overrides the QDockWidget::event() method to ignore such spontaneous show or hide events so QDockWidget won't emit the visibilityChanged() signal then.