Only persist the editing context if there's been actual semantic changes

[pcdavid]

Currently we persist the whole editing context any time a handler returns a ChangeKind.SEMANTIC_CHANGE.

Although it's not the common case, it's possible for a handler to return this ChangeKind even though the tool invoked did not actually perform any change in the semantic data (indeed, most tools' behavior is opaque to the handler which invoke them, so the handlers always assume the "worst" and suppose the tool did make some changes).

EMF has native support for resource modification tracking (resource.setTrackingModification(true) on load, and then resource.isModified() to test the flag and resource.setModified(false) to reset it on save), so identifying if any of the project's resources has actually been modified is trivial.

This is a simpler and safer subset of #3637.