Provide a signal to catch tabbed QDockWidget activation by it's tab activation

Currently it's impossible to detect tabbed QDockWidget activation moment.
There isn't a user-accessed event when a click on the dock tab is occured or the tab switched in other way (example: change the focus with Tab button and press arrow right/left button).

The signal can have a similar signature:

void QMainWindow::dockWidgetActivated(QDockWidget* w);

As QMainWindow has a set of tab bars inside that handle dock widgets switches, it's possible to forward a signal
QTabBar::currentChanged(int index) to the outside with replacing an integer parameter with a pointer to the activated QDockWidget.

See QMainWindowTabBar inside qmainwindowlayout.cpp