Codechange: use better location for the "invalid" action sentinel value

It used to be a random sentinel for end-of-(widget-)list that was used to tell
that no action has taken place yet. Since the last action is practically the
widget that was pressed, add the sentinel to that enumeration.

src/widgets/terraform_widget.h

@@ -22,6 +22,8 @@ enum TerraformToolbarWidgets {
 	WID_TT_PLANT_TREES,                       ///< Plant trees button (note: opens separate window, no place-push-button).
 	WID_TT_PLACE_SIGN,                        ///< Place sign button.
 	WID_TT_PLACE_OBJECT,                      ///< Place object button.
+
+	INVALID_WID_TT = -1,
 };
 
 /** Widgets of the #ScenarioEditorLandscapeGenerationWindow class. */
@@ -42,6 +44,8 @@ enum EditorTerraformToolbarWidgets {
 	WID_ETT_DECREASE_SIZE,                       ///< Downwards arrow button to decrease terraforming size.
 	WID_ETT_NEW_SCENARIO,                        ///< Button for generating a new scenario.
 	WID_ETT_RESET_LANDSCAPE,                     ///< Button for removing all company-owned property.
+
+	INVALID_WID_ETT = -1,
 };
 
 #endif /* WIDGETS_TERRAFORM_WIDGET_H */