Add all_steps and unique_steps property/function to relevant objects

[aekoch]

Currently, these functions that only exist at the GherkinProject level, where they are manually computed. It should be possible to do these functions at the Feauture and Scenario level.

Note: there is no one uniqueness algorithm, sometimes you want to find all steps with different raw texts, sometimes you want real texts, sometimes you want to ignore the keyword altogether, and sometimes you want to use the step with scenario outline parameters replaced. The unique_steps function should support using different step comparison strategies.

AC: The following functions/properties should exist and work:

• scenario.all_steps
• scenario.unique_steps(strategy)
• feature.all_steps
• feature.unique_steps(strategy)
• project.all_steps
• project.unique_steps(strategy)

These functions should also be added to any additional objects between Scenario and GherkinProject