Stop Leaking json Through Headers and Drop Includes Kept Only for the Name

AppConfig.hpp, DeviceManager.hpp and UserManager.hpp carried a global
"using namespace nlohmann;", json_diff.hpp a global "using json =
nlohmann::json;" and PrinterFileSystem.h a global "using nlohmann::json;".
Every file that included one of them, directly or not, could write a
bare json, and 63 did without declaring it.

The last two also made the include checker treat json_diff.hpp and
PrinterFileSystem.h as the headers that provide json, so they were
included from files that use nothing else from them: 57 of the 59
includers of json_diff.hpp never name json_diff.

The five statements are removed. Headers that use the type now spell
nlohmann::json, source files declare their own "using json =
nlohmann::json;", and the includes that only supplied the name are
dropped or replaced by <nlohmann/json.hpp>.

Eight files reached json_diff.hpp only through an include that is now
gone and with it lost that header's "using namespace std;". The std
names they used unqualified are qualified.

src/slic3r/GUI/DeviceCore/DevNozzleSystem.h

@@ -4,7 +4,7 @@
 
 #include "libslic3r/CommonDefs.hpp"
 #include "libslic3r/MultiNozzleUtils.hpp"
-#include "slic3r/Utils/json_diff.hpp"
+#include <nlohmann/json.hpp>
 
 #include "libslic3r/PrintConfig.hpp"
 #include <string>
@@ -195,6 +195,6 @@ namespace Slic3r
    {
    public:
        static void  ParseV1_0(const nlohmann::json& nozzletype_json, const nlohmann::json& diameter_json, DevNozzleSystem* system, std::optional<int> flag_e3d);
-       static void  ParseV2_0(const json& device_json, DevNozzleSystem* system);
+       static void  ParseV2_0(const nlohmann::json& device_json, DevNozzleSystem* system);
    };
 };