Scan source files in well-defined order.

Sort the filelist so that the order is predicable and results in
diff-friendly PO files.
Restore compatibility with GTK+ 2 on Unix.

wxWidgets' GTK+3 port is not quite ready for prime time yet and (almost?)
no distribution uses it.
Fix opening of files from welcome screen under GTK+.

Previously, a new window would open, instead of reusing the welcome
window.  This was because at the time of the UnusedActiveWindow() call,
the file dialog was the active window, thus preventing the welcome
screen from being found.
Fix assert when importing into TM on OS X.

The fix is a gross hack, but it's the only thing that can be done
quickly, without reimplementing wxProgressDialog.
Update WinSparkle, use the new shutdown API.

Implement WinSparkle's restart API: quit Poedit automatically after
launching the installer.  Refuse to do it if there's an unsaved file.
Update French translation.
Update Simplified Chinese translation.
Update Hungarian translation.
Update Azerbaijani translation.
Update Brazilian Portuguese translation.
Update Lithuanian translation.
Update Dutch translation.
Tweak translation memory matching algorithm.

Reduce the number of returned results, eliminate low quality ones:
discard hits with low score and if there's a phrase (exact or almost
exact) match, don't even try to generate lower-quality results.  Also
limit results to those with similar source text length.
Increase version to 1.6.4, update NEWS.