WIXFEAT:4258 - complete introduction of access modifiers for identifiers.

Completes the introduction of access modifiers for identifiers.
That is one way to look at it. Another way to look at it is:

1. It is less likely you need a File Id attribute since collisions (should) only happen when one installs the same filename to the same folder.
2. Thus the File Id attribute is used only for files that are referenced elsewhere.

In other words, if there is a File Id, the file is likely special (duplicated or referenced). That seems less complicated in the larger scheme of things.