Do not move/re-use notification widgets when hidden

Current problem:
"Notification" widget created as a top level 'tool-tip' like widget
After 10 seconds the widget is moved into the sub-widget hierarchy (under the "NotificationWidget") and works as a usual widget (not a tooltip) after that.

This causes different problems on different OSes because tooltip widgets are supported differently and one of the reason of the Windows test flakiness: the tooltip-level widget competes with context menu to be a 'topLevelPopupWidget' in QT.

The solution is simple: do not re-parent but re-create the "Notification" widget when it is moved from the floating to the embedded state