Keyboard Events chapter: Clarify keyboard event propagation.

* docs/tutorial/C/gtkmm-tutorial-in.xml: Mention that keyboard events are
first tested for keyboard shortcut keys.
* examples/book/keyboard_events/simple/keyboard_events.[h|cc]:
on_key_press_event() is an overridden default handler. Don't call connect().
* examples/book/keyboard_events/propagation/event_propagation.[h|cc]:
Add on_key_release_event() and windowKeyReleaseBefore().
Bug #661857, comment 22.