Fix minimal required CMake version
Add a class to handle dictzip files

Provides pseudo-random access to dictionary files compressed using dictzip.

It doesn't implement a cache, it just loads missing chunks until it has the
whole file.  I'm not sure if discarding not recently used chunks is really
a useful feature.  If there _was_ a way to get noticed when system memory
is low, I think the best way to handle that event would be to simply release
it all.

All in all, this is pretty useless.  But it was interesting to write.

This has yet to be integrated into the application proper.
Move private data into StardictDictPrivate
Actually use DictzipInputStream in sdtui
Fix the dictionary loading error path