Open quick help reference links as Dash URLs.

Rather than merely searching for a term extracted from the link information,
we now open the link directly in Dash, which allows us to disambiguate
between similar results and even return a result if a simple text search were to fail.

We open the link by preparing a URL according to the `dash-advanced://`
scheme, consisting of:

    1. The name of the result/page to show (e.g. NSAlert)
    2. The type of the result (e.g. Class)
    3. The path to open (e.g. the docset URL)

Per @kapeli, given this URL, Dash will then:

    1. Perform a regular search for the result using the currently enabled docsets
    2. Search for a result that matches the path
    3. If a result is found, that result will be prioritised and it will appear as the top result
    4. If a result with that path is not found, a fake result will be added to the top