core/settings: Move configuring_global behind an API

Rather than have directly modified global state here, we can make it an
implementation detail and have an interface that changes are queried
through.

src/core/settings.h

@@ -33,8 +33,6 @@ enum class CPUAccuracy {
     DebugMode = 2,
 };
 
-extern bool configuring_global;
-
 template <typename Type>
 class Setting final {
 public:
@@ -198,13 +196,18 @@ struct Values {
 
     // Add-Ons
     std::map<u64, std::vector<std::string>> disabled_addons;
-} extern values;
+};
 
-float Volume();
+extern Values values;
+
+bool IsConfiguringGlobal();
+void SetConfiguringGlobal(bool is_global);
 
 bool IsGPULevelExtreme();
 bool IsGPULevelHigh();
 
+float Volume();
+
 std::string GetTimeZoneString();
 
 void Apply();