Allow Dialog:path() to select a folder

[recatek]

For batch-export scripts, it would be useful if Dialog:path() could be scripted to select a folder, rather than just a single file.

[PKGaspi]

I think this is duplicated with #14. Nevertheless, this feature would be really useful. For now, the workaround is to use Dialog:file() and only use the dirname of that. The user would have to specify a file when asked for a folder tho, which is weird.