Fix not working maxima backend, due to a missing file

Summary:
Maxima backend don't work: you run entry, but calculation never ends. It's happend, because backend use special lisp configuration file (cantor-initmaxima.lisp) for better output processing but the file don't exist, bacause it have be removed in 616dc4159c7bbeeca962af58adbcccd025ddc723, I think, accidentally. So, If you have this file (from previous cantor installation, for example) all is okey, but if you don't, you can't run maxima entries. So I have recovered the file.

But there is some change, compared Application/15.12: Output is plain text, instead nice LaTeX image. I think, it bug too and i will try solve it later.

Test Plan:

1. remove purge cantor installation
2. check, that cantor-initmaxima.lisp have be removed from share/cantor/maximabackend
3. reinstall cantor without the patch
4. check, that cantor-initmaxima.lisp don't be contained in share/cantor/maximabackend
5. check, that maxima backend don't calculate
6. apply patch and reinstall cantor
7. check, that cantor-initmaxima.lisp exist in share/cantor/maximabackend
8. check, that maxima work normally

Reviewers: Cantor, asemke

Reviewed By: Cantor, asemke

Subscribers: asemke, Cantor, KDE Edu

Tags: KDE Edu, Cantor

Differential Revision: https://phabricator.kde.org/D11771