Add support for an all vars backup of Nspire calculators

[debrouxl]

Suggested by fghsgh on IRC.

Design and implementation notes:

• .recv_all_vars_backup is currently wired to tixx_recv_all_vars_backup in calc_nsp, but that won't work beyond one file: tifiles_group_contents bails out when encountering FileContent with CALC_NSPIRE model;
• before lifting that limitation (and similar ones at other places), a specification needs to be created: mainly how to represent the hierarchy of files and folders (with > 1 level of nesting) received from a calculator without losing information, and be able to faithfully transfer such a hierarchy to a calculator. ZIP files akin to tigroups, probably containing folders, are probably a path of least resistance, given that libtifiles already contains support for ZIP files. But someone also needs to check whether TI's software contain code to receive, produce, consume and send such a backup - in which case TI's format, assuming it fulfills the functional requirements (we can reasonably hope so ^^), should be adopted.