Ordering for K2 item attachments

[pinta83]

Here is my pull request to allow attachments to be ordered in the backend Item edit and shown by that ordering in the frontend/backend, related to https://github.com/getk2/k2/issues/493.

Since i don't use front editing in Joomla at all, I don't know if accompanying changes should be done for the frontend aswell.

IMPORTANT: Also have in mind that the extra "ordering" column in the k2_attachments table should be added to the update process of K2. This column should be created on update.

[fMalecki]

I would appreciate this feature as well! Would be great to have a possibility to order "related items" as well – actually, related items are ordered by default (ID) with no possibility to change. Being able to change the default ordering would help, too...

[jameswadsworth]

I'd like to see this feature too.