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/CMakeLists.txt

@@ -2,8 +2,8 @@ set(SOURCES
     Color/ColorConverter.cpp
     Color/ColorPrimaries.cpp
     Color/TransferCharacteristics.cpp
-    MatroskaDemuxer.cpp
-    MatroskaReader.cpp
+    Containers/Matroska/MatroskaDemuxer.cpp
+    Containers/Matroska/Reader.cpp
     PlaybackManager.cpp
     VideoFrame.cpp
     VP9/BitStream.cpp