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/CMakeLists.txt

@@ -626,6 +626,7 @@ set(SOURCE_FILES ${SOURCE_FILES}
 
         # rawinput
         rawinput/rawinput.cpp
+        rawinput/rawinput_handles.cpp
         rawinput/midi.cpp
         rawinput/sextet.cpp
         rawinput/piuio.cpp