Adding a property named Item on a content type causes problems

Description

We should make sure to throw an exception at initialization time if someone created a property named Item on a content type. See related bug.

Steps to reproduce
1. Create a new Alloy site.
2. Create a new property with name Item. For example, in the standardpage like this:

[Display(
GroupName = SystemTabNames.Content,
Order = 320)]
public virtual string Item
{ get; set; }

3. Try to run the site.
4. You get an error.

Adding a property named Item on a Block type causes no problems initially when using them as a local block. We should still make the same fix for block, because it causes problems in many cases (for example, when using it as a dynamic block).