Langtool compatible with GNU gettext format

[Garry Yao]

Enable the langtool to emit PO files instead of JS files.

[Garry Yao]

The second patch resolves missing entries from js language files instead of based on equivalence comparison with source language file.

[Wiktor Walc]

I did not check it yet completely, but there's already one thing that I do not agree with.

I'd definitely avoid deleting directory without asking for confirmation. Even if there is one chance in a million that someone will pass the wrong destination directory, someone will finally do it and will be very unhappy ;-)

Let's simply stop the execution when destination directory exists. If anyone wants to delete the destination directory automatically, he can do this in a batch script.

[Garry Yao]

Ok...just guess the language output directory always requires frequent updates, so let's leave it to bash/shell.

[Wiktor Walc]

I've committed Garry's patch with few enhancements with [7081].

[Garry Yao]

Committed [7083] to allow update language folder from PO translation files.