Implement Download folder path retrieval on OS X

The current implementation returns the DocumentLocation folder.
Since now only cocoa is supported, we can use NSFileManager to get the
correct path.

[ChangeLog][QtCore][OS X] Now QStandardPaths returns the correct path
for the DownloadLocation.

Change-Id: Ic0ea3ebf8585a1e34a7b43c734df78fd3949d4d4
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>