Add application events for indicating start/finish of restoring session

[EugeneZelenko]

Hi, Matthias!

It'll be great to add Application events that indicate start/finish of restoring state . I'd like to tweak layout views loaded from session, so relevant callback could be installed/uninstalled easily. Alternative requires to keep track of all layout views.

[klayoutmatthias]

I have added two new events to MainWindow (because that is the object that does the session restore): on_session_about_to_be_restored and on_session_restored.

[EugeneZelenko]

Hi, Matthias!

Thank you for help!