widget_selected
Emitted when a widget is clicked while a tips query is active. After the click, the query is stopped.
The widget set by set_caller() does not emit this signal; it stops the query immediately.
Note that the callback is called twice: When the user clicks the button, and when he releases it.
Callback function
void callback(GtkTipsQuery tips_query, GtkWidget widget, string tip_text, string tip_private, GdkEvent event);
忘却曲線を使ってこの知識を確実に記憶に残す
