Properties addable when working on a step

Currently, step properties can only be added statically to a step. It is not possible to add a property to a step when executing it. It shall be possible to preconfigure properies which can optionally be added to a step by the person who is actioning it. It should be possible to preconfigure whether a property is already added or is addable once or repeatedly. Therefore, the <tns:property> tag in propertyTemplates.xml shall be extended by a property addable which can accept those three states:

added − the property will already be visible (current behaviour)
once − the property can be added exactly once by someone working on the step
repeatedly − the property can be added repeatedly by someone working on the step