[Python] Massive refactoring
- Remove communication via DBus, replaced by KProcess
- Supports text result for a python expression with plot image result
- Show numpy arrays in full form (because we had solved problem with showing big strings in the variable model)
- Use Session::setVariableModel instead of handling variable model by self
- Better interrupt
- Use expression queue, model updating and expression finishing from Session
- Remove unused PythonSession members
- Some tests improvments
Closes T6113, T6114