Adding a timeout feature: sleep after some inactivity time

[josuah]

This was suggested on Discord by @Akitaishi
Currently the Noa app lets the Monocle run out of battery when not in use.
An alternative would be to get the Monocle to sleep, which could allow a day-long usage.
That would allow the Monocle to be woken-up on tap and immediately start listening a query, then go to sleep shortly after.
Not sure how to make a difference between wake-from-charging-case and wake-from-touch-button.
Maybe some smart needs to be added to the monocle firmware for that.

[josuah]

Cross-post with brilliantlabsAR/noa-for-ios#63