Sandbox: Create save-related temporary files in appropriate location.

Use NSItemReplacementDirectory on OS X to determine the location,
instead of just appending something to the filename.  This is not only
generally the right thing to do, but it has the benefit of being sandbox
friendly as well.
Sandbox: save MO files as related items.

Do the Cocoa dance necessary for the MO files alongside POs so that we
are allowed to save them too under the OS X sandbox.
Update wx: fix wxFileDialog under sandbox.
Merge branch 'refs/heads/osx-sandbox-fixes' into stable