Add a converter / importer for older lsp files

[pappou99]

I have a bunch of showfiles, which I'm not able to open with the newer Versions of LISP.(< 0.6)

Please add a possibility to import older files, so LISP is backwards compatible or provide informations how to convert these older files.

[FrancescoCeruti]

There's been a few discussion about this, with 0.6 LiSP saves the app and plugin version in the session file, the next step is to convert the file automatically to newer version (with a notice for the user).

I might be able to provide 0.5 -> X conversion when it's added