client: Copy file attributes on push/pull

Copy and move operations preserve file attributes (such as modification time) in most of existing scenarios: local copy/move, remote copy/move, file_copy_fallback in glib. However, one case remains special: copy/move between local and remote (gvfs) locations. It's implemented by push and pull operations in backends, which don't attempt to preserve the usual attributes (e.g., sftp doesn't preserve mtime). This commit implements a generic solution that covers missing functionality. A failure to set metadata is not fatal, resembling the behavior in the existing cases.

Closes: #346 (closed) Signed-off-by: Maxim Mikityanskiy maxtram95@gmail.com