audio: Move supported URI checking to mopidy.audio.utils

In order to avoid gstreamer imports leaking into more of our code I'm moving
this to a new utils class in audio.

mopidy/audio/utils.py

@@ -0,0 +1,23 @@
+from __future__ import unicode_literals
+
+import pygst
+pygst.require('0.10')
+import gst
+
+
+def supported_uri_schemes(uri_schemes):
+    """Determine which URIs we can actually support from provided whitelist.
+
+    :param uri_schemes: list/set of URIs to check support for.
+    :type uri_schemes: list or set or URI schemes as strings.
+    :rtype: set of URI schemes we can support via this GStreamer install.
+    """
+    supported_schemes= set()
+    registry = gst.registry_get_default()
+
+    for factory in registry.get_feature_list(gst.TYPE_ELEMENT_FACTORY):
+        for uri in factory.get_uri_protocols():
+            if uri in uri_schemes:
+                supported_schemes.add(uri)
+
+    return supported_schemes

mopidy/backends/stream/actor.py

@@ -1,15 +1,12 @@
 from __future__ import unicode_literals
 
-import pygst
-pygst.require('0.10')
-import gst
-
 import logging
 import urlparse
 
 import pykka
 
 from mopidy import settings
+from mopidy.audio import utils
 from mopidy.backends import base
 from mopidy.models import SearchResult, Track
 
@@ -24,15 +21,8 @@ class StreamBackend(pykka.ThreadingActor, base.Backend):
         self.playback = base.BasePlaybackProvider(audio=audio, backend=self)
         self.playlists = None
 
-        available_protocols = set()
-
-        registry = gst.registry_get_default()
-        for factory in registry.get_feature_list(gst.TYPE_ELEMENT_FACTORY):
-            for uri in factory.get_uri_protocols():
-                if uri in settings.STREAM_PROTOCOLS:
-                    available_protocols.add(uri)
-
-        self.uri_schemes = list(available_protocols)
+        self.uri_schemes = utils.supported_uri_schemes(
+            settings.STREAM_PROTOCOLS)
 
 
 # TODO: Should we consider letting lookup know how to expand common playlist