Emitted when the apply button is clicked.
The default behavior of the `GtkAssistant` is to switch to the page after the current page, unless the current page is the last one.
A handler for the apply signal should carry out the actions for which the wizard has collected data. If the
action takes a long time to complete, you might consider putting a page of type gtk_assistant_page_progress
after the confirmation page and handle this operation within the [[email protected]:GtkAssistant:prepare] signal
of the progress page.