[changes.exsd] Removed some references to now-deleted interfaces

The validator and executor elements are now essentially just duplicates of
their descriptor-based counterparts
[IChangeDescriptor] Removed the checked exception from the simulate method

As an added bonus, this makes it possible to remove the overridden version
of this method from the IChange interface
Removed all (ab)uses of the java.lang.Error class

Java's guidelines for unchecked exceptions make it clear that all of these
cases should have been handled with RuntimeExceptions
[Change] Deleted Change

Although it has no methods or fields left, IChange will probably remain (as
a tag for deprecated old changes) for a while longer
[ModelObject] Removed the ModelObjectChange class

The former subclasses of ModelObjectChange now just implement IChange
directly
[PropertyScratchpad] Deleted the executeChange method
[BoundDescriptor] Implement IChangeDescriptor instead of IChange

Even once IChanges have been removed completely, the ability to bind an
IChangeDescriptor to a Resolver will still be useful
[HandlerUtilities] Simplified the name validity checking methods

The separation between IChange and IChangeDescriptor handling is no longer
necessary
[ChangeCommand] Removed a now-erroneous reference to IChange