core, citra_qt: add frame advancing to framelimiter

Frame advancing is a commonly used TAS feature which basically means running the game frame by frame. TASers use this feature to press exact buttons at the exact frames. This commit added frame advancing to the framelimiter and two actions to the Movie menu. The default hotkey is `\` for advancing frames, and `Ctrl+A` for toggling frame advancing. The `Advance Frame` hotkey would automatically enable frame advancing if not already enabled.

src/core/perf_stats.cpp

@@ -74,6 +74,13 @@ double PerfStats::GetLastFrameTimeScale() {
 }
 
 void FrameLimiter::DoFrameLimiting(microseconds current_system_time_us) {
+    if (frame_advancing_enabled) {
+        // Frame advancing is enabled: wait on event instead of doing framelimiting
+        frame_advance_event.Wait();
+        frame_advance_event.Reset();
+        return;
+    }
+
     if (!Settings::values.use_frame_limit) {
         return;
     }
@@ -104,4 +111,20 @@ void FrameLimiter::DoFrameLimiting(microseconds current_system_time_us) {
     previous_walltime = now;
 }
 
+void FrameLimiter::SetFrameAdvancing(bool value) {
+    const bool was_enabled = frame_advancing_enabled.exchange(value);
+    if (was_enabled && !value) {
+        // Set the event to let emulation continue
+        frame_advance_event.Set();
+    }
+}
+
+void FrameLimiter::AdvanceFrame() {
+    if (!frame_advancing_enabled) {
+        // Start frame advancing
+        frame_advancing_enabled = true;
+    }
+    frame_advance_event.Set();
+}
+
 } // namespace Core