Widgets and Quick hybrid UI development: Make sure that focus handling works

Focus/activation-related bugs observed when embedding a QQuickWindow in a widget window:

• Focus gets stuck inside embedded window and has no way to go back to parent window
• Some controls (eg: combobox) inside the embedded window are painted in an inactive state when that window has not yet gained focus, even if the parent (widget)window is focused
• When the focus moves from the embedded window to the parent window, focus rect remains on the focus object of the embedded window
• The next time the embedded window gains focus, the focus will resume from the last focus object, which is wrong behavior. The embedded window's focus object should be cleared when it looses focus
• When first tabbing from widget to window container, the focus won't immediately go to the item inside the container. It will require another tab key for it to gain focus. This doesn't happen the next times.