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/MediaFilePanel.cpp

@@ -45,6 +45,8 @@
 #include "../Utils/MacDarkMode.hpp"
 #endif
 
+using json = nlohmann::json;
+
 namespace Slic3r {
 namespace GUI {