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/rawinput/hotplug.cpp

@@ -54,7 +54,7 @@ namespace rawinput {
                     switch (hdr->dbch_devicetype) {
                         case DBT_DEVTYP_DEVICEINTERFACE: {
                             auto dev = (const DEV_BROADCAST_DEVICEINTERFACE *) hdr;
-                            this->ri_mgr->devices_scan_midi();
+                            this->ri_mgr->midi_scan_start();
 
                             // check if class is not HID
                             if (memcmp(&dev->dbcc_classguid, &GUID_HID, sizeof(GUID_HID)) != 0)
@@ -112,6 +112,7 @@ namespace rawinput {
                             std::string name(dev->dbcc_name);
 
                             // destruct device
+                            log_misc("hotplug", "device interface removal: {}", name);
                             this->ri_mgr->devices_remove(name);
                         }
                     }
@@ -120,8 +121,11 @@ namespace rawinput {
                     return TRUE;
                 }
                 case DBT_DEVNODES_CHANGED: {
-                    // TODO: this can be a little noisy as it gets called on every device
-                    this->ri_mgr->devices_scan_midi();
+                    // catch-all device-tree change: MIDI and XInput have no targeted
+                    // arrival/removal notification, so this is our only hook to detect them
+                    // can be a little noisy as it gets called on every device
+                    log_misc("hotplug", "device tree changed, rescanning MIDI + XInput");
+                    this->ri_mgr->midi_scan_start();
                     this->ri_mgr->devices_scan_xinput();
                     break;
                 }