Fix invalid minimum size for translated messages in option button

[Dragoncraft89]

computing the minimum size was done on the untranslated messages. However, when auto translation is enabled these will most likely not match their translated counterparts which leads to layout bugs

[KoBeWi]

Makes sense, but the text should probably be cached, to avoid translating it often when not necessary. Check PopupMenu, its Item subclass has xl_text property. OptionButton could get something like that too.