Fix crash when closing a document with visible infobars

Apparently the ::unrealize symbol is sent too late, after we destroyed
the widget and freed the GeanyDocument, and some signals like
::focus-in-event can still be fired on the widget after that.

So, properly use the ::destroy signal that is supposed to be fired when
others should release references to that instance.

src/document.c

@@ -3215,7 +3215,7 @@ static gboolean on_sci_key(GtkWidget *widget, GdkEventKey *event, gpointer data)
 static void enable_key_intercept(GeanyDocument *doc, GtkWidget *bar)
 {
 	/* automatically focus editor again on bar close */
-	g_signal_connect_object(bar, "unrealize", G_CALLBACK(gtk_widget_grab_focus), doc->editor->sci,
+	g_signal_connect_object(bar, "destroy", G_CALLBACK(gtk_widget_grab_focus), doc->editor->sci,
 			G_CONNECT_SWAPPED);
 	g_signal_connect_object(doc->editor->sci, "key-press-event", G_CALLBACK(on_sci_key), bar, 0);
 }