The user-defined signal handlers are called callbacks.
The first parameter the callback receives is the widget that
emitted the signal. The rest are signal-dependent. For example, the callback
for switch_page signal of GtkNotebook class would look like this:
<?php function on_switch_page($notebook, $page, page_num) { } ?>
where $page is the object representing the page that was switched to,
and $page_num is its page number.
The low-level event callbacks receive an additional object describing
the event. It has properties for event type, button number if it was a mouse
click, key code if it was a key press, etc.
It is possible to connect more than one callback to the same signal.
They will be run in the order they are connected. It is also possible to
simulate emission of a signal by the widget, for some custom purposes.
