Refactoring the DataSource.

Now we have a new class for DataSource to enable management of
Operations on Data Sources.
WIP: Adding support for operators.
Crude first pass at adding support for Python expression/operators.
Working on operators panel.
Added UI to add/edit Python operators.
Fix segfault on exit.
Fix build errors after rebase.
Merge remote-tracking branch 'operators'

Conflicts:
	CentralWidget.cxx