rawinput: handle midi scanning asynchronously, fix midi hotplug (#793)

## Link to GitHub Issue or related Pull Request, if one exists
Fixes #603

## Description of change
Scanning for midi devices can take a while on Windows 11 with MIDI 2.0
service. Sometimes it'll take a couple seconds. Sometimes, on a PC with
zero MIDI devices, it takes 10-11 seconds.

This was causing two issues:

1. slow startup time
2. crash on invalid memory access due to a race condition

Address both.

Also fix a bug: when a device change event fires, we do a MIDI scan, and
invalidated all existing MIDI devices in favor of creating new handles.
Stop doing this, and instead check for duplicates by matching the ID and
keep existing device handles alone. Properly clean up devices on unplug.

Also: fix MIDI buttons being stuck on when unplugged while holding a
key.

## Testing
Tested with Nostroller in MIDI mode. rtpMIDI works too.

src/spice2x/cfg/api.cpp

@@ -411,6 +411,12 @@ GameAPI::Buttons::State GameAPI::Buttons::getState(rawinput::RawInputManager *ma
                         BUTTON_PRESSED : BUTTON_NOT_PRESSED;
                     break;
                 }
+                case rawinput::DESTROYED:
+                    // device was unplugged. release the button instead of leaving
+                    // state at getLastState(), which would latch a held button on
+                    // forever (e.g. a MIDI note held while the device is removed)
+                    state = BUTTON_NOT_PRESSED;
+                    break;
                 default:
                     break;
             }