Reusable XPath Constraint Snippets

Ability to create reusable XPath constraints. This avoids

1. Duplication of query logic across multiple places
2. Unmaintainable XPath definitions when adding new features to an app - lots of duplicated XPaths may need updating
3. Confusing XPath logic when conditions become more advanced

Creating reusable snippets allows one place to manage these conditions, and can therefore be updated in one place. It also reduces the cognitive load when building XPaths from scratch each time.