Support Dash's docsets

[data-man]

https://kapeli.com/dash for macOS is has a lot excellent documentation sets for offline using.
Of course, there is the open-source Zeal.
But Zeal unpack tgz-based docsets to an user's home directory.
It would be nice to have support these tgz in goldendict-ng without unpacking them if it's possible.

[AnabasisXu]

Second to this. It would be really nice if we can browse Zeal docsets within GoldenDict-ng

But Zeal unpack tgz-based docsets to an user's home directory.

Oh, I just found that you can now custom docsets dir:

But I agree that unpacking them leads to a huge number of files...my PC is too slow even to count the files in the docsets folder.