Some simplifications and fixes to showing local tile ID

* Removed separate updating function and data member
* Use TileLayer::contains instead of TileLayer::bounds().contains()
* Don't show 0 as "None" since for the local tile ID, 0 means the first
  tile in the tileset
* Tweaked notation a little
* Removed unnecessary isEmpty and isTileLayer checks
* Removed the isVisible check, since I think it is a little confusing to
  display "None" just because the layer isn't visible
* Made sure the text is translatable, and renamed "None" to "empty"