Alter Holdings Table?

The Holdings table already showed who is holding the shares (Contact_Infos_ID) but it also needs to show which company's shares are being held. So I added Listed_Companies_ID. I also updated the unique constraint.

BTW, the procedures for accepting buy and sell offers will also need to be changed, if this pull request is merged. I didn't change the procedures at all.

Please pull if you approve of these changes.