device: Add simple temperature model for devices

This temperature model has three states:
 * COLD
 * WARM
 * HOT

Device drivers can define the time it requires for the device to get HOT
and COLD. The underlying model assumes an exponential warming and
cooling process and enforces a cool-off time after the device has
reached the HOT state. This cool down period is however shorter than the
specified time in the driver.

libfprint/drivers/synaptics/synaptics.c

@@ -1385,6 +1385,7 @@ fpi_device_synaptics_class_init (FpiDeviceSynapticsClass *klass)
   dev_class->scan_type = FP_SCAN_TYPE_PRESS;
   dev_class->id_table = id_table;
   dev_class->nr_enroll_stages = ENROLL_SAMPLES;
+  dev_class->temp_hot_seconds = -1;
 
   dev_class->open = dev_init;
   dev_class->close = dev_exit;