Support new version of 'Write field on linked issues or subtasks' JWT post-function

The way this post-function parameters are stored in the XML has changed in recent version of JIRA Workflow Toolbox plugin (JWT), so that PC did not detect them and the parameter values were not being correctly translated between instances.

Support has also been added for similar parameters in post-functions ReadIssuesJQL and ReadLinkedIssues.

The new format is similar to the Create Issue post-function referred in -PCDEV-644-.