Add setting to prefer uni or pair sources and fix selection b/w them

pair_quality != uni_quality was the wrong condition to check,
since there are cases where the target_resolution is 360, and
there are no pair sources at 360, but there are some at other
resolutions, which would falsely select the pair sources.

Signed-off-by: Jesús <heckyel@hyperbola.info>

settings.py

@@ -168,6 +168,15 @@ For security reasons, enabling this is not recommended.''',
         'category': 'playback',
     }),
 
+    ('prefer_uni_sources', {
+        'label': 'Prefer integrated sources',
+        'type': bool,
+        'default': True,
+        'comment': '',
+        'category': 'playback',
+        'description': 'If enabled and the default resolution is set to 360p or 720p, uses the unified (integrated) video files which contain audio and video, with buffering managed by the browser. If disabled, always uses the separate audio and video files through custom buffer management in av-merge via MediaSource.',
+    }),
+
     ('use_video_player', {
         'type': int,
         'default': 1,