Adopt existing namespaces into OpenRCT2 namespace (#22368)

* Put all of TitleSequenceManager into the same namespace

* Move RideConstructionState into the OpenRCT2 namespace

* Adopt existing namespaces into OpenRCT2 namespace

This adds `using namespace OpenRCT2` to compilation units where appropriate,
as a means to get the codebase to compile until these units have been placed
in a namespace of their own.

src/openrct2/platform/Platform.Linux.cpp

@@ -37,7 +37,7 @@
 #    include "../localisation/Language.h"
 #    include "Platform.h"
 
-namespace Platform
+namespace OpenRCT2::Platform
 {
     std::string GetFolderPath(SPECIAL_FOLDER folder)
     {
@@ -391,6 +391,6 @@ namespace Platform
         return path;
     }
 #    endif // NO_TTF
-} // namespace Platform
+} // namespace OpenRCT2::Platform
 
 #endif