ZEN-4176 Prevent validation from running multiple times

[tfesenko]

The "Update KaiZen Editor validation markers" job now cancels other validations on the current file if a new validation is initiated.

In general, validation is invoked:

• Inside a bigger WorkspaceJob on save(), saveAs(), and validation preferences change for Swagger v2
• As an individual WorkspaceJob "Update KaiZen Editor validation markers" on document opening (setInput()) and change(documentChanged())

Extracted a lightweight ValidationOperation which:

• Cancellable
• Provides more accurate progress report