Better integration with `ConstructionBase`

[mattsignorelli]

parameter group definitions, including properties which are not fields, should be more consistent with how ConstructionBase works so that Accessors can play nicely with promotion.

e.g. overriding Base.propertynames and trying to do an Accessors.set gives:

ERROR: The `RectangleParams` type defines custom properties: it has `propertynames` overloaded.
Please define `ConstructionBase.setproperties(::RectangleParams, ::NamedTuple)` to set its properties.

This will also make it more consistent with e.g. only settable vs only gettable properties.

Note that properties != "virtual properties" which are only at the line element level.