Update jsoncpp from version 0.5.0 to 1.9.5

Aside from including a ton of bug fixes, this should fix an issue in the
CI pipeline when building with system libraries that was caused by a
minor incompatibility between 0.5.0 and less ancient versions.

Also, switch to the amalgamated source, because it makes it easier to
keep it up to date.
https://github.com/open-source-parsers/jsoncpp/wiki/Amalgamated-(Possibly-outdated)

src/ScreenInstallOverlay.cpp

@@ -5,7 +5,7 @@
 #include "Preference.h"
 #include "RageLog.h"
 #include "FileDownload.h"
-#include "json/value.h"
+#include "json/json.h"
 #include "JsonUtil.h"
 #include "SpecialFiles.h"
 class Song;