GUI2: made more use of retval::NONE over 0

src/gui/dialogs/modal_dialog.cpp

@@ -28,7 +28,7 @@ namespace dialogs
 {
 modal_dialog::modal_dialog()
 	: window_(nullptr)
-	, retval_(0)
+	, retval_(retval::NONE)
 	, always_save_fields_(false)
 	, fields_()
 	, focus_()

src/gui/widgets/button.cpp

@@ -44,7 +44,7 @@ button::button(const implementation::builder_button& builder)
 	: styled_widget(builder, get_control_type())
 	, clickable_item()
 	, state_(ENABLED)
-	, retval_(0)
+	, retval_(retval::NONE)
 {
 	connect_signal<event::MOUSE_ENTER>(
 			std::bind(&button::signal_handler_mouse_enter, this, _2, _3));

src/gui/widgets/menu_button.cpp

@@ -41,7 +41,7 @@ menu_button::menu_button(const implementation::builder_menu_button& builder)
 	: styled_widget(builder, get_control_type())
 	, selectable_item()
 	, state_(ENABLED)
-	, retval_(0)
+	, retval_(retval::NONE)
 	, values_()
 	, selected_()
 	, keep_open_(false)

src/gui/widgets/multimenu_button.cpp

@@ -42,7 +42,7 @@ REGISTER_WIDGET(multimenu_button)
 multimenu_button::multimenu_button(const implementation::builder_multimenu_button& builder)
 	: styled_widget(builder, get_control_type())
 	, state_(ENABLED)
-	, retval_(0)
+	, retval_(retval::NONE)
 	, max_shown_(1)
 	, values_()
 	, toggle_states_()

src/gui/widgets/toggle_button.cpp

@@ -39,7 +39,7 @@ toggle_button::toggle_button(const implementation::builder_toggle_button& builde
 	: styled_widget(builder, get_control_type())
 	, state_(ENABLED)
 	, state_num_(0)
-	, retval_(0)
+	, retval_(retval::NONE)
 	, icon_name_()
 {
 	connect_signal<event::MOUSE_ENTER>(std::bind(

src/gui/widgets/toggle_panel.cpp

@@ -41,7 +41,7 @@ toggle_panel::toggle_panel(const implementation::builder_toggle_panel& builder)
 	: panel(builder, get_control_type())
 	, state_(ENABLED)
 	, state_num_(0)
-	, retval_(0)
+	, retval_(retval::NONE)
 {
 	set_wants_mouse_left_double_click();