Added 'focus_on_window_activation' directive

Added 'focus_on_window_activation' directive

When a window receives a _NET_ACTIVE_WINDOW message, it can steal the focus. This may not be preferable to all users.
With this directive, the user can choose from one of the following:
1) 'smart' - focus the container if its workspace is visible, otherwise set the urgency flag (default)
2) 'urgent' - always set the urgency flag, do not steal focus
3) 'focus' - always switch focus, never set the urgency hint
4) 'none' - ignore the request entirely (do not switch focus, nor set the urgency hint)

fixes #1426