The point of this patch is to fix a couple of inconsistencies in `viewer.html`, compared to the locale files, such that the viewer would work correctly even without the `l10n/` files present.
*Note:* Since this isn't changing any of the locale files, we should *not* need to update any of the string names.
Looking through the history of the findbar code, it seems that the `findPrevious`/`findNext` buttons have never had a `title` set (note PR 2168, which was the initial findbar implementation).
Furthermore, the `placeholder` of the `findInput` didn't agree 100% with locale file either, so this is also adjusted.