Make csync_file_stat_t public and partly convert to C++

Make csync_file_stat_t public and partly convert to C++

This is the first commit trying to unify csync_file_stat_s,
csync_vio_file_stat_s and csync_tree_walk_file_s. Use QByteArray
and unique_ptr already since I'm not used to track memory allocations
and this will make the transition easier.

Issue #1817