#609 Review registration for keyboard notifications

[Christa Runge]

The following issue was seen at svn 3881:
In the current implementation, some classes register for three keyboard notification types:
UIKeyboardWillShowNotification, UIKeyboardWillChangeFrameNotification, UIKeyboardWillHideNotification.
Other classes only for two of these.

Should be reviewed if this is intentional. If yes, add comments with details.