Removed unnecessary interface

The IMacroService interface did not add anything to the solution so I
removed it. Everything one would like to mock in the case of testing is
dependency injected into the service any way.
Save as and bug fix

Added "Save As" support and fixed a bug in the "Open" functionality.
Added error checking and comments

Added a null check on the repository in the SaveChanges method that
takes a file name.

Added a comment to the same method.
Pimped SaveDialog

Added an XML-restriction to the save dialog and removed the override
prompt.