textscreen: Set parent pointer for conditionals.

We rely on the parent pointer to determine if the widget is being
hovered over. Set the parent pointer so that the background is set
properly for a widget in a conditional container.

textscreen/txt_conditional.c

@@ -141,6 +141,8 @@ txt_conditional_t *TXT_NewConditional(int *var, int expected_value,
     conditional->expected_value = expected_value;
     conditional->child = child;
 
+    child->parent = &conditional->widget;
+
     return conditional;
 }