LibVideo: Reorganize demuxer file hierarchy and rename Matroska files

As new demuxers are added, this will get quite full of files, so it'll
be good to have a separate folder for these.

To avoid too many chained namespaces, the Containers subdirectory is
not also a namespace, but the Matroska folder is for the sake of
separating the multiple classes for parsed information entering the
Video namespace.

Userland/Libraries/LibVideo/Containers/Demuxer.h

@@ -0,0 +1,39 @@
+/*
+ * Copyright (c) 2022, Gregory Bertilson <zaggy1024@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/NonnullOwnPtr.h>
+#include <LibCore/Object.h>
+#include <LibVideo/DecoderError.h>
+#include <LibVideo/Sample.h>
+#include <LibVideo/Track.h>
+
+namespace Video {
+
+class Demuxer {
+public:
+    virtual ~Demuxer() = default;
+
+    virtual Vector<Track> get_tracks_for_type(TrackType type) = 0;
+
+    DecoderErrorOr<NonnullOwnPtr<VideoSample>> get_next_video_sample_for_track(Track track)
+    {
+        VERIFY(track.type() == TrackType::Video);
+        auto sample = TRY(get_next_sample_for_track(track));
+        VERIFY(sample->is_video_sample());
+        return sample.release_nonnull<VideoSample>();
+    }
+
+    virtual DecoderErrorOr<void> seek_to_most_recent_keyframe(Track track, size_t timestamp) = 0;
+
+    virtual Time duration() = 0;
+
+protected:
+    virtual DecoderErrorOr<NonnullOwnPtr<Sample>> get_next_sample_for_track(Track track) = 0;
+};
+
+}