#76 Invoices module product deletion patch

[Kristofer Widholm]

This patch will allow you to delete products from an
invoice on a 2.0.4 installation of dotproject. I
discovered it didn't work after the upgrade, realized I
had to overload the CProduct->delete() method.

The invoices module appears to be orphaned. There are a
few different modifications to the CVS tree version out
there in the wild, so this patch might be seamless for
everyone.

[Kristofer Widholm]

Invoices module fix for broken product deletion permissions