Re #2389. Added workspace index attribute to TabulatedFunction.

Changed it to be able to load any loadable file. Resolution inherits from TabulatedFunction.
Re #2389. Correction to handling empty string attributes.
Re #2389. Added StringEditorFactory to QtPropertyBrowser.

The created editors update properties only when user finishes editing.
Re #2389. Added parameter to pass workspace index to fitting function.
Re #2389. Added unit tests for new property of PlotPeakByLogValue.
Re #2389. Added a checkbox to sequential fit dialog to pass ws index.

Also some clean-up of Convolution code.
Re #2389. Changed Resolution class inheritance.
Resolution mustn't call TabulatedFunction::functionDeriv1D method.

Re #2389
Merge remote-tracking branch 'origin/feature/2389_resolution_sequential_fitting'