Rename 'Settings' to 'ViewHistory'

This PR renames `Settings` to `ViewHistory`, as was suggested by @brendandahl [previously](https://github.com/mozilla/pdf.js/pull/3255#issuecomment-26374851).

Beside the renaming, this patch also removes `isLocalStorageEnabled` to reduce unnecessary code duplication, since this function is available in [ui_utils.js](https://github.com/mozilla/pdf.js/blob/master/web/ui_utils.js#L259).