widgettitle value

```widgettitle``` doesn't really have any value here or in core. Proposing to change to ```widget-title``` instead and maybe add that into stylesheet too. 

The class is available only in /widgets.php and not present in any of the stylesheet's, at least I could not locate one. As in my previous proposal, this is for the consistency reasons only.
Merge pull request #499 from emiluzelac/patch-21

Change `widgettitle` to `widget-title` in 404 template for better consistency throughout all theme files. _s uses `widget-title` as its convention when declaring theme widgets, so we'll go with that.