[Headlines and Enclosures] Remove hard-coded entries in favour of configuration files

[sjehuda]

Enclosures

The selection of local download managers would better be configurable with configuration files, preferably with .desktop files.

See also #1388

Headlines

The selection of oblibe annotation managers would better be configurable with configuration files, preferably with .desktop files, or, if needed, OpenSearchDescription.

<OpenSearchDescription>
<ShortName>DankSearX</ShortName>
<Description>a privacy-respecting, hackable metasearch engine</Description>
<InputEncoding>UTF-8</InputEncoding>
<Image>
https://searx.danksquad.org/searx/static/themes/simple/img/favicon.png?aadae686b35ad9af5975d2623d397808d0e5db35
</Image>
<LongName>searx metasearch</LongName>
<Url rel="results" type="text/html" method="get" template="https://searx.danksquad.org/searx/search?q={searchTerms}"/>
<Url type="application/opensearchdescription+xml" rel="self" template="/searx/opensearch.xml?method=POST&autocomplete="/>
</OpenSearchDescription>

Source: https://searx.danksquad.org/searx/opensearch.xml?method=POST&autocomplete=