This avoids the static initialization order fiasco between Core and VideoCommon

Co-authored-by: Jordan Woyak <jordan.woyak@gmail.com>

Use the normal state changed `HookableEvent` instead of having
`Core::NotifyStateChanged` call `g_perf_metrics.OnEmulationStateChanged`
directly.

The direct call was added in bad78cfed416d89ceb6b0acf6c134d3691b3d624 to
avoid a crash. At the time state changed callbacks were stored in a
vector, and the crash was caused by `g_perf_metric`'s destructor trying
to remove the callback from the already-destroyed vector.

Later a97627e736ff352e031123519c799e65cc98b32c switched state changed
callbacks to use `HookableEvent`, which is specifically designed to
handle the case where a hook outlives its associated event.

Since the workaround is no longer necessary replace it with a standard
`EventHook`.

Previously, PerformanceTracker registered a callback to be updated on
emulation state changes. PerformanceTrackers live in a global variable
(g_perf_metrics) within libvideocommon. The callback was stored in a
global variable in libcore. This created a race condition at shutdown
between these libraries, when the PerfTracker's destructor tried to
unregister the callback.
Notify the PerfTracker directly from libcore, without callbacks, since
Core.cpp already references g_perf_metrics explicitly. Also rename
Core::CallOnStateChangedCallbacks to NotifyStateChanged to better
reflect what it's doing.

PerformanceTracker: Eliminate mutex. General cleanups.