Don't show slug metabox on woo shop_orders and shop_coupon pages

[erichk4]

The slug metaboxes might not be necessary for the woocommerce orders and coupon pages...

[herrvigg]

Looks good to me.
@spleen1981 please could you review? As you have integrated WC with slugs you could have better feedbacks.

I had to rewrite without the null coalescing operator as this is PHP7.0 and QTX still allows PHP 5.4+. The requirements will be raised soon, but until then the retro-compatibility is needed.

[spleen1981]

Both post and post_type are not guaranteed to exist in $_GET, so they would need to be tested to avoid warnings.
Also post_type seems not to be the right key to test in this case.
Anyways global $post_type seems a cleaner test here.
I think also that is better to move the logic inside the callback, there is no real advantage doing otherwise and readability is worst IMO.
Also I think is better to have an array of "not applicable types" there, as I suspect others may come out.
The above implemented here edcb9ac

@herrvigg as a side note these post types should not be translatable at all (ref. also #1116), we should check upstream and grey-up unchecked the relevant post types checkboxes in qtranslate advanced settings.