Tidying up Truth[Decay]CollectionMaker following migration to R/W/D handles

This MR makes a few refinements to Truth[Decay]CollectionMaker following the migration to R/W/D handles, namely:

• passing event context also to decorators
• header-only declaration/initialisation of properties
• using Gaudi::Properties for all non-handle properties
• initialisation of decoration names with whatever input collection name is provided, to account for the (unlikely) possibility of a truth collection other than TruthParticles being required