flatpak: reworked packages + made a building script: a new `build_manifest.py` script can now be used to generate flatpak manifests for every frontend of SàT. The manifest can be used both for development versions and stable ones. Templates files (in the form `_tmp_<app-id>.json`) are used to set building instructions. A common runtime specific to SàT has been abandoned following a discussion on the official mailing list. A small wrapper is now used to launch backend automatically if it's not found. Desktop and app metadata have been added for Cagou. Jp and Primitivus don't have appdata and desktop files yet.

// Big Integer Library v. 5.5
// Created 2000, last modified 2013
// Leemon Baird
// www.leemon.com
//
// This file is public domain.   You can use it for any purpose without restriction.
// I do not guarantee that it is correct, so use it at your own risk.  If you use 
// it for something interesting, I'd appreciate hearing about it.  If you find 
// any bugs or make any improvements, I'd appreciate hearing about those too.
// It would also be nice if my name and URL were left in the comments.  But none 
// of that is required.