"Missing" date filter and argument

Many many people have been confused by the fact that the Date filter and argument show up in the 'Date' group instead of the 'Content' group. This is because they can be used for non-CCK date fields as well as date fields and you can't put a filter or argument into more than one group.

In the process of working on the D7 port I stumbled across a way to get the date filter and argument to show up in both places. More precisely -- they still show up in the 'Date' group but the 'Content' values now also show up.

Getting ready to do a commit.