core_timing: Convert core timing into a class

Gets rid of the largest set of mutable global state within the core.
This also paves a way for eliminating usages of GetInstance() on the
System class as a follow-up.

Note that no behavioral changes have been made, and this simply extracts
the functionality into a class. This also has the benefit of making
dependencies on the core timing functionality explicit within the
relevant interfaces.

src/core/hle/kernel/kernel.h

@@ -12,8 +12,9 @@ template <typename T>
 class ResultVal;
 
 namespace Core::Timing {
+class CoreTiming;
 struct EventType;
-}
+} // namespace Core::Timing
 
 namespace Kernel {
 
@@ -39,7 +40,11 @@ public:
     KernelCore& operator=(KernelCore&&) = delete;
 
     /// Resets the kernel to a clean slate for use.
-    void Initialize();
+    ///
+    /// @param core_timing CoreTiming instance used to create any necessary
+    ///                    kernel-specific callback events.
+    ///
+    void Initialize(Core::Timing::CoreTiming& core_timing);
 
     /// Clears all resources in use by the kernel instance.
     void Shutdown();