Defer registering SaveParticipant until IWorkspace service is available

[kwin]

This closes #356

What it does

How to test

Author checklist

• [x] I have thoroughly tested my changes
• [x] The change is following the coding conventions
• [x] I have signed the Eclipse Contributor Agreement (ECA)

[kwin]

@laeubi Can you have a look as you merged the similar PR https://github.com/eclipse-platform/eclipse.platform/pull/752? Thanks.

[laeubi]

I'm not a JDT committer.

[kwin]

Maybe you can have a look @akurtakov ?