This allows to copy a property list and then selectively
replace various properties. e.g.
```
PropertyList plist (region->properties ());
plist.add (Properties::length, len);
plist.remove (Properties::start);
```
See also 8b0ab38675
and 97f0fac7d5
This also fixes the issue referenced in
8c83149c4c
ebf59d4426