Fix focus changing when unminimising on X11

Fix focus changing when unminimising on X11

In f220e3b0783a24a6c7195f170297cf4b12a29d85 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.

BUG: 497803