Plugin should be able to define named selectors for Behat tests

Behat has a number of named selectors that make it easier to write readable and robust checks, for example:

When I click on "Preview" "link" in the "My first pattern match question" "table_row"

However, this list is currently fixed in core Moodle. Similar to MDL-48024, it should be possible for all plugins to contribute named selectors, so one could write things like

When I click on "Reply" "link" in the "Hello world" "mod_forum/post"

(that naming convetion is just a suggestion, but we already assume that Behat tests authors know Frankenstyle component names, for the @mod_forum tags we expect them to to .feature files.)