Layered listing searchable text adapter lookup

[ramonski]

Description of the issue/feature this PR addresses

This PR changes the signature for listing searchable text adapters to include the request layer as well:

text_providers = getAdapters((instance, api.get_request(), catalog),
                             IListingSearchableTextProvider)

This avoids an adapter registration clash with add-ons that are not installed:

For: ('adapter', (<InterfaceClass bika.lims.interfaces.IAnalysisRequest>, <InterfaceClass senaite.core.interfaces.catalog.ISampleCatalog>), <InterfaceClass bika.lims.interfaces.IListingSearchableTextProvider>, u'')
    File "senaite/patient/catalog/indexer/configure.zcml", line 10.2-14.55
        <adapter
            for="bika.lims.interfaces.IAnalysisRequest
                 senaite.core.interfaces.ISampleCatalog"
            provides="bika.lims.interfaces.IListingSearchableTextProvider"
            factory=".sample.ListingSearchableTextProvider"/>
    File "my.lims/src/my/lims/adapters/configure.zcml", line 8.4-8.72
          <adapter factory=".searchable_text.ListingSearchableTextProvider" />

Current behavior before PR

Searchable text adapters are called from any add-on (even if not installed).

Desired behavior after PR is merged

Searchable text adapters are called only for active add-ons and are registered with the request as well.

--
I confirm I have tested this PR thoroughly and coded it according to PEP8
and Plone's Python styleguide standards.

[xispa]

Thanks! I suggest to create another PR for the definitive removal of the call to non-layered adapters, for 2.6.0