GtkPlacesSidebar: Fix Home and Desktop on windows.

When concatenating "file://" to a path (Returned from g_get_user_special_dir())
not always get a correct uri..

On unix: g_get_user_special_dir() return ig: "/home/matias" and concatenating
you get "file:///home/matias", that is a correct uri.

On windows: g_get_user_special_dir() return ig: "c:\users\matias\" and
concatenating you get "file://c:\user\matias", but the correct uri is:
"file:///c:\user\matias"