GtkWidget::grab_focus
This method sets the initial grab and focus on the calling widget so
that, for example, if a GtkText were to call it,
any keyboard events would be picked up immediately by the text widget.
grab_focus() can only be called
when the widget in question has the GTK_CAN_FOCUS
flag set. Some will have this set at source, others will not, and will
generate an error message to that effect when the method is called.
Use set_flags() to override the
source settings.
