Rework device removal to have a nice API

This enhances the device removal to create a well defined behaviour.

Primarily, it means that:
 * "device-removed" will only be called for closed devices
 * "removed" will be called only when no operation is active

Note that all actions will fail with FP_DEVICE_ERROR_REMOVED, *except*
for open which will only return this error if it failed.

Resolves: #330

libfprint/fp-device-private.h

@@ -29,6 +29,7 @@ typedef struct
   GUsbDevice  *usb_device;
   const gchar *virtual_env;
 
+  gboolean     is_removed;
   gboolean     is_open;
 
   gchar       *device_id;