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
Put eyeball on the right, indent the pipeline widget

Simpler than I thought in the end, we might want to add our own model
in the end, and use the model-view framework, but this looks like we
want and only needed a few lines of code to be modified.
Merge branch 'indent-pipeline-widget'
Fix "Deleting unknown object: TomoOptions" warning.

This warning shows up when VTK_DEBUG_LEAKS is ON.
Added a new View properties panel.