Allow extensions to hijack i18n_product_type_alert

This allows extensions to remove or hijack the display of the product type alert introduced with SHA: 519de5be59684.

A product may have variations, but not be explicitly `'variable'`. For example, a variable subscription.

At the moment, the product type error will be displayed every time the page loads for any product that has variations but is not core's variable product.

I thought WooCommerce could allow extensions to hijack the warning by using `triggerHandler()`. Specifically, replacing:

```
if ( 'variable' !== select_val && 0 < $( '#variable_product_options input[name^=variable_sku]' ).length ) {
```

With: 

```
if ( 'variable' !== select_val && 0 < $( '#variable_product_options input[name^=variable_sku]' ).length && $( 'body' ).triggerHandler( 'woocommerce-display-product-type-alert' ) !== false ) {
```

However, the `$( 'select#product-type' )` change event is triggered when the script is loaded. That's a good thing, but for this circumstance, it means any script trying to attach itself to 
`'woocommerce-display-product-type-alert'` to turn off the alert needs to be loaded *before* the `'wc-admin-meta-boxes'` script (i.e. `meta-boxes-product.js`). This doesn't make sense for most scripts because they probably depend on the WC script.

So to make it work, I moved the alert logic outside of the default `change` handler and attached it to the custom `'woocommerce-product-type-change'` event.

This allows extensions to hijack the alert for their product types with code like: 

```
	$( 'body' ).on( 'woocommerce-display-product-type-alert', function(e, select_val) {
		if (select_val=='variable-subscription') {
			return false;
		}
	});
```

It also has the added benefit of allowing the alert to be removed completely with `.off()`.