Bug 1040651 - Map a preference to a Gaia setting to enable Layerscope

Add a new option, "LayerScope", in "Settings -> Developer", so we can
enable this perference dynamically, instead of enabling it manually by
edit-prefs.sh.
Hey, @BorisChiou, let me try to clarify, since this is a new thing that we're promoting in new Gaia code.  You don't need to write `Dump layerscope` in the HTML.  All that is needed is the `data-l10n-id="dumps-layaerscope"` part. See https://developer.mozilla.org/en-US/Firefox_OS/Developing_Gaia/localization_code_best_practices#Using_a_declarative_API for more information.  Thanks!