GtkPlacesSidebar: Fix Home and Desktop on Windows.
GtkPlacesSidebar: Fix Home and Desktop on Windows.

We were concatenating "file://" to the result of g_get_user_special_dir() to build
a URI, but this is not enough on Windows.  Use g_filename_to_uri() instead.

See https://bugzilla.gnome.org/show_bug.cgi?id=739453

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