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.h

@@ -91,6 +91,7 @@ typedef enum {
  * @FP_DEVICE_ERROR_DATA_NOT_FOUND: Requested print was not found on device
  * @FP_DEVICE_ERROR_DATA_FULL: No space on device available for operation
  * @FP_DEVICE_ERROR_DATA_DUPLICATE: Enrolling template duplicates storaged templates
+ * @FP_DEVICE_ERROR_REMOVED: The device has been removed.
  *
  * Error codes for device operations. More specific errors from other domains
  * such as #G_IO_ERROR or #G_USB_DEVICE_ERROR may also be reported.
@@ -106,6 +107,8 @@ typedef enum {
   FP_DEVICE_ERROR_DATA_NOT_FOUND,
   FP_DEVICE_ERROR_DATA_FULL,
   FP_DEVICE_ERROR_DATA_DUPLICATE,
+  /* Leave some room to add more DATA related errors */
+  FP_DEVICE_ERROR_REMOVED = 0x100,
 } FpDeviceError;
 
 GQuark fp_device_retry_quark (void);