Refactor ChangeUtil: universal API converting Change to WorkspaceEdit

[Eskibear]

Currently for each type of Change, there's a corresponding public method located in ChangeUtil, which is not elegant or consistent. E.g.

• void convertCompositeChange(Change change, WorkspaceEdit edit)
• void convertResourceChange(ResourceChange resourceChange, WorkspaceEdit edit)
• void convertTextChange(WorkspaceEdit root, IJavaElement element, TextChange textChange)
• etc.

Anytime you want to convert a Change to a WorkspaceEdit, you have to explicitly call the corresponding utility based on the type of Change.

This PR refactors the utility and use ChangeUtil.convertToWorkspaceEdit(Change change) as the only entry to convert Change to WorkspaceEdit.

[testforstephen]

Looks good to me.