Fixes and improvements (#616)

TactilityKernel/include/tactility/device_listener.h

@@ -8,6 +8,7 @@ struct Device;
 
 enum DeviceEvent {
     DEVICE_EVENT_STARTED,
+    DEVICE_EVENT_STOPPING,
     DEVICE_EVENT_STOPPED,
 };
 

TactilityKernel/source/device.cpp

@@ -256,6 +256,8 @@ error_t device_stop(Device* device) {
     internal->state.stopping = true;
     unlock_internal(internal);
 
+    device_listener_notify(device, DEVICE_EVENT_STOPPING);
+
     error_t unbind_error = driver_unbind(internal->driver, device);
 
     lock_internal(internal);