order item count is wrong

Order item count is wrong when using ```size_of``` and comes out unfiltered. This had been fixed at some point by introducing ```order->get_item_count```, which takes quantities into account *and* filters the output.

In addition to this:

1. It would be very helpful to filter the whole string displayed in the column, to allow including additional information about item count, where this need arises (see comment below).

2. By filtering the tr class, we can customise the styling of the item list (related to https://github.com/woothemes/woocommerce/pull/5663).

Please consider adding this into the next point release! Thanks :)