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