Fix multiple listeners being attached to models (#192)

Fixes wintercms/winter#1251 by replacing all instances of bindEvent inside a Laravel Model Event by bindEventOnce.

There remains an edge case if the event gets cancelled by another listener with a higher priority.

> Using bindEventOnce would fix this issue in most cases, but there's one I can think of where it would fail: if someone registers a handler for model.afterSave with more priority than the default, and this handler sometimes returns false.
> In this case, let's imagine we save the model a first time: we bind the afterSave() method once; we fire the event; it gets cancelled by the other handler; so we don't call the afterSave() method.
> Then we save a second time: we bind afterSave() again (we now have 2); we fire the event; for some reason the other handler does not return false; we call afterSave() twice.

src/Database/Model.php

@@ -228,7 +228,7 @@ protected function bootNicerEvents()
                     if ($model->methodExists($method)) {
                         // Register the method as a listener with default priority
                         // to allow for complete control over the execution order
-                        $model->bindEvent('model.' . $method, [$model, $method]);
+                        $model->bindEventOnce('model.' . $method, [$model, $method]);
                     }
                     // First listener that returns a non-null result will cancel the
                     // further propagation of the event; If that result is false, the

src/Database/Traits/SoftDelete.php

@@ -34,7 +34,7 @@ public static function bootSoftDelete()
             if ($model->methodExists('beforeRestore')) {
                 // Register the method as a listener with default priority
                 // to allow for complete control over the execution order
-                $model->bindEvent('model.beforeRestore', [$model, 'beforeRestore']);
+                $model->bindEventOnce('model.beforeRestore', [$model, 'beforeRestore']);
             }
             /**
              * @event model.beforeRestore
@@ -54,7 +54,7 @@ public static function bootSoftDelete()
             if ($model->methodExists('afterRestore')) {
                 // Register the method as a listener with default priority
                 // to allow for complete control over the execution order
-                $model->bindEvent('model.afterRestore', [$model, 'afterRestore']);
+                $model->bindEventOnce('model.afterRestore', [$model, 'afterRestore']);
             }
             /**
              * @event model.afterRestore

src/Database/Traits/Validation.php

@@ -65,7 +65,7 @@ public static function bootValidation()
             if ($model->methodExists('beforeValidate')) {
                 // Register the method as a listener with default priority
                 // to allow for complete control over the execution order
-                $model->bindEvent('model.beforeValidate', [$model, 'beforeValidate']);
+                $model->bindEventOnce('model.beforeValidate', [$model, 'beforeValidate']);
             }
 
             /**
@@ -87,7 +87,7 @@ public static function bootValidation()
             if ($model->methodExists('afterValidate')) {
                 // Register the method as a listener with default priority
                 // to allow for complete control over the execution order
-                $model->bindEvent('model.afterValidate', [$model, 'afterValidate']);
+                $model->bindEventOnce('model.afterValidate', [$model, 'afterValidate']);
             }
 
             /**

src/Halcyon/Model.php

@@ -242,7 +242,7 @@ protected function bootNicerEvents()
                     if ($model->methodExists($method)) {
                         // Register the method as a listener with default priority
                         // to allow for complete control over the execution order
-                        $model->bindEvent('model.' . $method, [$model, $method]);
+                        $model->bindEventOnce('model.' . $method, [$model, $method]);
                     }
                     // First listener that returns a non-null result will cancel the
                     // further propagation of the event; If that result is false, the