Switch to PO files for translations

The new scripts are written in Python. They handle conversion from RC to POT/PO (conversion to PO shouldn't be needed in general unless I'm mistaken) and from PO to RC.

Each RC file correspond to 3 PO files:
 - one for the dialog strings (two identical English strings with the same dialog and component IDs must have the same translation),
 - one for the menu strings (two identical strings with the same ID must have the same translation),
 - one for the string table strings (two identical strings with the same ID must have the same translation).

I normalized semi-manually the RC files so that everything goes smooth for the first sync. However the inherent deduplication implied by the use of PO files might be sort of annoying. This is mostly true for the menu strings since it basically means that you can't have different menu access keys for the same menu entry used in different menus. This makes more difficult or even impossible to have completely optimal access keys.

The gain from this is that the scripts are much more resilient to file shuffling as demonstrated by 6af21feca7adf716bd0d806dcc92c7ee7771194a. Also I plan to try Transifex later today or tomorrow.

I think I'm mostly done with the work so I thought it was time to open a PR for discussion.