Added serialization of non-unique labels to project files.
The address labels are stored without the non-unique tag, because we
can get that from the file offset. (If we stored it, we'd need to
extract the value and verify that it matches the offset.) Operand
weak references are symbolic, and so do include the tag string.
We weren't validating symbol labels before. Now we are.
This also adds a "NonU" filter to the Symbols window so the labels
can be shown or hidden as desired.
Also, added source for a first pass at a regression test.
