Move most of the Meson build to subdirectories

We sadly cannot call `shared_library()` and `executable()` in these
subdirectories while still maintaining the same `build/` directory
structure, but this is still much cleaner. All of the other build
artifacts are now also gone from the root of `build/` so it's cleaner
overall.

src/common/serialization/vst3/event-list.cpp

@@ -16,7 +16,7 @@
 
 #include "event-list.h"
 
-#include "src/common/utils.h"
+#include "../../utils.h"
 
 YaDataEvent::YaDataEvent() noexcept {}
 

src/common/serialization/vst3/process-data.cpp

@@ -16,7 +16,7 @@
 
 #include "process-data.h"
 
-#include "src/common/utils.h"
+#include "../../utils.h"
 
 YaProcessData::YaProcessData() noexcept
     // This response object acts as an optimization. It stores pointers to the