Set HIDDEN attribute on .Sugar-Metadata directory

According to the OLPC AU dpeloyment [1], 50% of the support calls they
get from teachers are due to a confusion caused by looking in the
.Sugar-Metadata directory for files from Windows. "On Windows, the
directory is not hidden, and people are going into it expecting their
files to be in there. What's even more confusing is that the filenames
they are looking for are indeed inside, but those are metadata and not
the actual files."

This patch uses a new Sugar extension, sugar-fatattr, to set the
HIDDEN attribute on the .Sugar-Metadata directory.[2]

[1] http://sugardextrose.org/issues/1367
[2] https://github.com/sugarlabs/sugar-toolkit-gtk3/pull/33