Core: Re-write frame limiter

Now based on std::chrono, and also works in terms of emulated time
instead of frames, so we can in the future frame-limit even when the
display is disabled, etc.

The frame limiter can also be enabled along with v-sync now, which
should be useful for those with displays running at more than 60 Hz.

src/core/core.h

@@ -94,6 +94,7 @@ public:
     }
 
     PerfStats perf_stats;
+    FrameLimiter frame_limiter;
 
 private:
     /**