rawinput: avoid deadlock when binding analog axis as a button (#809)

## Link to GitHub Issue or related Pull Request, if one exists
Regressed by #793

## Description of change
Due to lock inversion, when binding an analog axis as a button, spice
deadlocks. Fix that.

Also create a separate `unordered_map` that keeps track of device
handles so that `WM_INPUT` handle can look up devices without having to
acquire the larger `devices_mutex` which could be held by (potentially)
lengthy operations like hotplug.

Fix more synchronization issues around hotplug. Latent bug exposed by
MIDI 2.0 issues.

## Testing

src/spice2x/rawinput/rawinput_handles.h

@@ -0,0 +1,28 @@
+#pragma once
+
+#include <mutex>
+#include <unordered_map>
+
+#include <windows.h>
+
+#include "device.h"
+
+namespace rawinput {
+
+    class RawInputHandles {
+    public:
+        struct AcquiredDevice {
+            Device *device = nullptr;
+            std::unique_lock<std::mutex> lock;
+        };
+
+        void add(Device *device);
+        void remove(Device *device);
+        AcquiredDevice acquire(HANDLE handle);
+
+    private:
+        // non-owning index; RawInputManager owns the stable device slots
+        std::unordered_map<HANDLE, Device *> devices;
+        std::mutex mutex;
+    };
+}