Can't add Part to ContentType when there is already Named Part added

[lampersky]

You can't add a Regular Part to a ContentType if there is already a Named Part of the same part added.

Steps to Reproduce:

• Create an attachable and reusable part, let's say MyTestPart,
• Create a ContentType or pick exisiting one,
• Add a Named Part (MyTestPart) and assign a name to it,
• Try to add a Regular Part (MyTestPart),
• MyTestPart is not listed, so you can't add it.

Workaround:

• Remove the Named Part (MyTestPart),
• Add the Regular Part (MyTestPart) first,
• Add the Named Part (MyTestPart) again.