Re #5151. Get the required values from the ConvertToMD algorithm.

This removes the use of MDEvents in CustomInterfaces and will allow us
to drop the dependency.
The item being retrieved is the same as the allowed values on the
ConvertToMD properties that this is in the end being used to set.
Re #5151. Remove inclusion of (unused) DataObjects header.

Also remove other unused headers and move one to the places where it is
actually being used.
Re #5151. Remove uses of DataObjects classes.

We can use the API interfaces IPeak(sWorkspace) instead, though I did
have to add one method to the IPeaksWorkspace interface.
Re #5151. Remove dependencies on MDEvents and DataObjects.
Re #5151. Add a unit test for CreateMDWorkspaceAlgDialog.

This is to ensure that the polling of the ConvertToMD algorithm for
property allowed values continues to work.
Also put the class itself into the correct namespace.
Merge branch 'feature/5151_remove_custominterfaces_dependencies' into develop