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

• Add support for constants to Trait Gen
• 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.