Fix menu/journal/book/scroll buttons when using localised MW-installations

This patch implements a custom widget that can switch its texture on-the-fly, making it obsolete having to use an atlas to get a hover animation. This also removes the predefined size restriction and should now work with all button texture sizes.

Obsoletes #388 http://bugs.openmw.org/issues/388
Fixes #386 http://bugs.openmw.org/issues/386