Properties for Packages

Hi,

16243 add property API to RPackage
        https://pharo.fogbugz.com/f/cases/16243


testPropertyAtPut

        | testValue package |
       
        testValue := Date today.
        package := self class package.
       
        package propertyAt: #testKeySelector put: testValue.
        self
                assert: (package propertyAt: #testKeySelector)
                equals: testValue.

        package removeProperty: #testKeySelector.
        self assert: package properties isNil.


(and yes, the property API should be a trait…)