Provide Retina artwork.

Where necessary (window-close and the Validate toolbar button), redo the
existing one so that the two versions match.
OS X: Fix layout and rendering issues on Retina displays.
Install dump-legacy-tm.exe.
Don't use MMapDirectory on Windows.

Looks like memory-mapped access isn't quite ready for prime time in
Lucene++ yet. Use the old fashioned SimpleFSDirectory.

Hopefully fixes #558.
Fix waiting for dump-legacy-tm termination on Unix.

The SIGCHLD signal would never be delivered in case of error.
Add (more) detailed logging of TM migration process.
Fix misdetection of empty PO files as invalid.
Increase version to 1.6.1, update NEWS.