Fix documented but not working "PropertyTask refid" feature

According to documentation `PropertyTask` supports an `refid` attribute to reference a previously defined property - but this currently ain't working.

The PR solve this issue and make the following example scenario possible:

Define properties e.g. in _build.properties_
```
my.property.definitons = 1,2
my.property.definiton.1 = foo
my.property.definiton.2 = bar
```

Dynamicly access properties in a build file
```xml
<foreach list="${my.property.definitons}" param="key" target="subtask" />

<!-- within target subtask -->
<property name="value" refid="my.property.definiton.${key}" />
<echo message="my.property.definiton.${key} is ${value}" />
```

**Note**
Adding reference within `Project::setPropertyInternal` should be reviewed, if it has negative side effects - even none occurred in my scenarios yet.