Provide a "do not switch off screen when on charge" option

[Matthew Exon]

It would be useful to have an option to keep the screen switched on and not blank while the phone is connected to the charger.

Use cases:

* I'm using the phone as a secondary screen while I work on something on my computer.
* I'm using the phone as a controller to my media system. I want it just sitting on the table so I can tell at a glance what's currently playing
* I'm using the phone as a timer while I exercise

Note that it is not necessary to keep the screen illumimated for most of these use cases. I would expect the screen to... uh... "deilluminate", is that a word?... after a few minutes, but I still want to be able to see what's on the screen. Perhaps an option to define a secondary illumination level for this case would be a good idea.

Suggested in the notes to Bug #1512211.