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.

PerformanceMetrics: Fix window arrangement

Fix overlays stacking on top of each other or not moving to the edge of
the screen when enabling or disabling overlays while emulation is
active.

This change only applies when Config::GFX_MOVABLE_PERFORMANCE_METRICS is
False.

- PPCSymbolDB:  Resolve shadowed variable warnings
- PerformanceMetrics:  Resolve shadowed variable warnings
- SWEfbInterface: Add missing declarations

PerformanceTracker: Eliminate mutex. General cleanups.

Clamp overlays to the render window (with some padding), reset their
positions when the render window changes sizes, and add a setting to
enable moving the overlays (off by default, .ini only for now).