Port Qalculate to QProcess

Summary:
* Set up the connection  between Cantor and qalc
* Use  streams for I/O
*  Parse the output returned by qalc
* Segregate API dependent parts fromqalc
* Use expression queue and command queue for commands that are to be processed by qalc
* use qalc for saving variables
* update variable model

Reviewers: filipesaraiva

Reviewed By: filipesaraiva

Subscribers: filipesaraiva, rishabhg

Tags: #cantor

Differential Revision: https://phabricator.kde.org/D6074