Fixes the build warning:

active-sensor.c: In function ‘active_sensor_new’:
active-sensor.c:425:44: warning: iteration 2 invokes undefined behavior [-Waggressive-loop-optimizations]
         active_sensor->alarm_timeout_id[i] = -1;
         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~
active-sensor.c:424:5: note: within this loop
     for (i = 0; i < NUM_NOTIFS; i++) {
     ^~~

implicit declaration of function ‘sensors_applet_conf_setup_sensors’

Added timer to active sensors, to suppress infinite popups, when a drive is removed from the system, with its sensor still active in msa.
fixes #62

Hold the value labels to the largest size they have been
unless the units have just been shown or hidden.