EmuScreen: Show the loading spinner while a savestate saves or loads

The boot spinner, in the same corner without the background, while a
save, load or rewind is queued or in progress. Only after a quarter of a
second, so it doesn't flash up on every quick save; on desktop that's
rarely reached, but slow devices get some feedback.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Core/SaveState.cpp

@@ -454,6 +454,12 @@ int g_screenshotFailures;
 		return !rewindStates.Empty();
 	}
 
+	bool IsBusy() {
+		std::lock_guard<std::mutex> guard(mutex);
+		// A save leaves the queue once its snapshot is taken, but stays pending until it's written.
+		return !g_pendingOperations.empty() || !g_pendingSaves.empty();
+	}
+
 	// Slot utilities
 
 	std::string AppendSlotTitle(const std::string &filename, const std::string &title) {