Changes

CellML Annotation view: fixed a class member access problem.
Indeed, we were trying to call 
CellmlAnnotationViewCellmlElementItem::type() on our model’s invisible 
root item, which obviously cannot work, even though it used to be
‘fine’ until now…?!

Some minor cleaning up.

Moved SimulationExperimentViewSimulationWidget::FileType to
SimulationExperimentViewSimulation::FileType.

Some minor cleaning up.

Some minor cleaning up.

CellML support: added a method to directly check for a CellML file's
validity.

Some minor cleaning up.

COMBINE support: now have methods to check that it is supported and to
retrieve its corresponding SED-ML file.

SED-ML support: allow the creation of a SED-ML with the file name of its
owner.
This will come handy later on when we can retrieve the corresponding 
CellML file of a SED-ML file.

SED-ML support: can now validate a SED-ML file directly.
This is in contrast with before where we could only validate some file 
contents.

Some minor cleaning up.

Moved
SimulationExperimentViewSimulationWidget::sedmlAlgorithmSupported() to
SedmlFile::algorithmSupported().

SED-ML support: now have methods to check that it is supported and to
retrieve its corresponding CellML file.
This has basically meant moving 
SimulationExperimentViewSimulationWidget::sedmlFileSupported() to 
SedmlFile::isSupported() and 
SimulationExperimentViewSimulationWidget::retrieveCellmlFile() to 
SedmlFile::cellmlFile().

Some minor cleaning up.

Simulation Experiment view: prevent a potential problem with filters
when exporting a simulation to SED-ML or a COMBINE archive.