Add InputText:setText and avoid non-intuitive UI behavior after light toggle

Discussed in #20. We could instead do something like that, which I think would be even more intuitive:
```Lua
fl:toggle()
self.fl_dialog.input:setText(fl:isOn() and ("%d"):format(self.intensity) or "")
```
But for this to work we would need to implement `isOn()` both for Kindle and Kobo. For the Kindle, it could be implemented readily in pure Lua, however for the Kobo we would need to change the kobolight module in koreader-base to expose its `isOn` variable (or track the state independently, which is ugly), but I prefer not do to that now, let's move kobolight to FFI first :P