Causes this to become the default widget.
this must be able to be a default widget; typically you would ensure this yourself by calling set_can_default with a true value. The default widget is activated when the user presses Enter in a window. Default widgets must be activatable, that is, activate should affect them. Note that Entry widgets require the “activates-default” property set to true before they activate the default widget when Enter is pressed and the Entry is focused.