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 616dc415, 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