Implement EBUR128Analysis

The most juicy bit!

This is based on Song Fingerprint Analysis,
but here we must know the actual song, and not just the file.

The library supports only interleaved S16/S32/F32/F64,
so we must be sure we insert `audioconvert` into pipeline.

One point of contention here for me, is whether we should
feed the frames to the library the moment we get them
in `NewBufferCallback`, or collect them in a buffer
and pass them all at once. I've gone with the former,
because it seems like that is not the worst choice:
https://github.com/strawberrymusicplayer/strawberry/pull/1216#issuecomment-1610075876

In principle, the analysis *could* fail,
so we want to handle that gracefully.

src/engine/ebur128analysis.h

@@ -0,0 +1,41 @@
+/*
+ * Strawberry Music Player
+ * Copyright 2023 Roman Lebedev
+ *
+ * Strawberry is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * Strawberry is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with Strawberry.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+#ifndef EBUR128ANALYSIS_H
+#define EBUR128ANALYSIS_H
+
+#include "config.h"
+
+#include <optional>
+
+#include "core/song.h"
+#include "ebur128measures.h"
+
+class EBUR128Analysis {
+ public:
+  ~EBUR128Analysis() = delete;  // Do not construct variables of this class.
+
+  // Performs an EBU R 128 analysis on the given song.
+  // Returns `std::nullopt` if the analysis fails.
+  //
+  // This method is blocking, so you want to call it in another thread.
+  static std::optional<EBUR128Measures> Compute(const Song &song);
+};
+
+#endif  // EBUR128ANALYSIS_H