Substitution constants in trait views.

[elliotcmorris]

As a result of discussion about this : #70

Add constants to generated traits that support substitution (primarily locatableContent) to be used with substitution functions as the "keys"

For example

std::string resolvedPath = "/image_{frame}.png";
openassetio::utils::substitute(resolvedPath, {LocatableContent::FRAME_TOKEN, 123});

See how FRAME_TOKEN is provided, rather than just having to type out "frame" or whatever...

Tasks

Beta Give feedback

1. Add support for constants to Trait Gen
2. Add string substitution constants to relevant traits

[elliotcmorris]

This has more implications that you'd think. Currently what the substitution variables are are just docced in the IsTemplated property, when really they should/need to be data that can be generated from.