feat: WailsShutdown callback

Now handles proper shutdown through:
  * runtime.Window.Close()
  * Killing the main window
  * Ctrl-C

lib/binding/method.go

@@ -18,6 +18,7 @@ type boundMethod struct {
 	log                *logger.CustomLogger
 	hasErrorReturnType bool // Indicates if there is an error return type
 	isWailsInit        bool
+	isWailsShutdown    bool
 }
 
 // Creates a new bound method based on the given method + type
@@ -39,6 +40,11 @@ func newBoundMethod(name string, fullName string, method reflect.Value, objectTy
 		err = result.processWailsInit()
 	}
 
+	// Are we a WailsShutdown method?
+	if result.Name == "WailsShutdown" {
+		err = result.processWailsShutdown()
+	}
+
 	return result, err
 }
 
@@ -211,3 +217,20 @@ func (b *boundMethod) processWailsInit() error {
 
 	return nil
 }
+
+func (b *boundMethod) processWailsShutdown() error {
+	// We must have only 1 input, it must be *wails.Runtime
+	if len(b.inputs) != 0 {
+		return fmt.Errorf("Invalid WailsShutdown() definition. Expected 0 inputs, but got %d", len(b.inputs))
+	}
+
+	// We must have only 1 output, it must be error
+	if len(b.returnTypes) != 0 {
+		return fmt.Errorf("Invalid WailsShutdown() definition. Expected 0 return types, but got %d", len(b.returnTypes))
+	}
+
+	// We are indeed a wails Shutdown method
+	b.isWailsShutdown = true
+
+	return nil
+}