Replace expression label with user property when possible

Expression is the more technical label for a field which should contain exaclty 1 expression, so you can add it as a -Dkey=value on the commandline.
Plugin developers often mix default-value and expression, but with the new Mojo Annotations this difference is much more clear by talking about property instead of expression and removing the ${ and }.
There are still enough expressions which contain a default-value ( for instance ${project.build.directory}/generated-sources/foobar/ ), so only in case of valid expressions the "User property" label should be used.