We have to regenerate previews when we change language. But we also need to do it
when translation is changed or added for a language, etc.
Previously, we were storing one preview per language, which was also stuffing
preview dir with (potentially) tens of PNGs per font file, if user plays with translations.
Now we use a better system, which is storing an additional optional metadata in previews
(some hexdigest), that Blender can use in addition to datetime to decide when to regenerate
previews.
This is only used (and needed) by font previews so far, but can easily be reused for other
types of previews if needed.
bec3131112