Add ability to set the station button urls

[ChriD]

The station buttons are defined in the content directory as far as i researched. (0/Renderers/[rendererUDN]/StationButtons)
Maybe we can change them with the content directory methods?

[ChriD]

Added methods assignStationButton(...) and getStationButtonAssignment(...) on media server whereby the getStationButton assignment will always return a reference?
Currently we may only assign valid container ids as objectId to the Buttons, but this is enough for now