feat: WailsShutdown callback

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

lib/interfaces/bindingmanager.go

@@ -7,4 +7,5 @@ type BindingManager interface {
 	Bind(object interface{})
 	Start(renderer Renderer, runtime Runtime) error
 	ProcessCall(callData *messages.CallData) (result interface{}, err error)
+	Shutdown()
 }

lib/interfaces/eventmanager.go

@@ -8,4 +8,5 @@ type EventManager interface {
 	Emit(eventName string, optionalData ...interface{})
 	On(eventName string, callback func(...interface{}))
 	Start(Renderer)
+	Shutdown()
 }

lib/interfaces/ipcmanager.go

@@ -5,4 +5,5 @@ type IPCManager interface {
 	BindRenderer(Renderer)
 	Dispatch(message string)
 	Start(eventManager EventManager, bindingManager BindingManager)
+	Shutdown()
 }