Fixed formatting of FrameRateInTrickMode.dlna.org to have 2-digit framerate

Changed PlaySpeedResponse to format the framerate as a 2-digit number,
 per DLNA 7.5.4.3.3.16.10.
Added support for adding the "Vary" header to responses when time-seek or play-speed are supported

Per DLNA 7.5.4.3.2.35.4, the Vary header needs to be added to resource
 responses when the resource/uri supports time-seek or playspeed.

Added virtual method HTTPGetHandler.supports_playspeed(). As with the
 other "supports*" methods, the default implementation returns "false".

Added HTTPMediaResourceHandler.supports_playspeed() override. This
 queries the associated MediaResource's supports_playspeed() method,
 which was pre-existing.

HTTPGet.handle_item_request() now adds the Vary header indicating the
 time-seek/playspeed header if/when the associated
 HTTPGetHandler.supports_time_seek()/supports_playspeed() returns true,
 respectively.

Moved the time-seek, playspeed, and cleartext request/response const header
 definitions into the respective class namespace. They probably shouldn't
 be defined in the top-level namespace. And didn't want to add to the
 crime with this change.

Ran some HEAD requests on resources with varying capabilities and
 confirmed value of Vary header.
Added Pragma response header for HTTP 1.0 and added HTTP 1.1 check for Vary response

HTTPTimeSeekResponse.DLNAPlaySpeedResponse.add_response_headers() now add the
 no-cache Pragma header for HTTP 1.0 requests (per DLNA .

The Vary header is now only included for HTTP 1.1 requests.

Logic in the HTTPRequest constructor to set the HTTP version of the response
 is relocated to the end of HTTPGet.handle_item_request().

 The way the libsoup apis work, there's no differentiation between the
 request HTTP version and the response HTTP version. So to allow the
 processing code to tailor its behavior to the HTTP version of the request
 (in accordance with DLNA 7.5.4.3.2.7.3), but unconditionally respond
 as HTTP 1.1 (in accordance with DLNA 7.5.4.3.2.7.2), the version must
 be set at the end of processing.

 This functionally was added via Gnome Bugzilla 702555. So requested
 it be revisited.

Tested with HTTP 1.0 and 1.1 requests with various PlaySpeed and
 TimeSeek headers and confirmed the correct Vary/Pragma headers are
 being added.