(1) ConfigurationWidget now uses its own display mode
(ConfigurationWidget::DisplayMode) instead of Qtilities::DisplayMode.
There is still a constructor for Qtilities::DisplayMode as well, so this
change won't break old code.
(2) Fixed issue in ConfigurationWidget where it did not always position
the widget in the center of the screen.
(3) Added 4 new modes to ConfigurationWidget: DisplayLeftModeWidgetView,
DisplayRightModeWidgetView, DisplayTopModeWidgetView,
DisplayBottomModeWidgetView.
(4) Fixed possible crashes in ObserverTreeModel.
(5) Some refactoring in DebugWidget.
(6) Some refactoring in ObserverWidget.
Some more polishing of ConfigurationWidget upgrades.