[Bangle] non js function with jswrap in the name

Yes, sometimes this is done in the code, although maybe it's a little confusing... The idea is ‎jswrap_banglejs_pwrBacklight is a function used by stuff in jswrap_banglejs.c, even if it's not actually a JS function.

At least if you see the symbol it in a listing of compiled code you know kind of where it's defined - if all you saw was pwrBacklight it wouldn't give you much hint of where it was defined.

Sometimes I use underscores for functions that are used like this, for example _jswrap_banglejs_setLocked is used by jswrap_banglejs_setLocked, but also a bunch of other functions, so maybe we could just do that in this case?