add a 'extract GSP template' refactoring to GSP editor

Suggestion for a new refactoring: to support the DRY principle it would be useful to have a 'extract GSP template' refactoring.

In a gsp file the user highlights the snippet to be extracted. When selecting the refactoring the following actions should occur:
- ask for the name of the gsp template to be created
- insert the snippet into a newly created template gsp
- remove the snippet from the current page by a `` tag

See [http://www.grails.org/doc/latest/guide/6.%20The%20Web%20Layer.html#6.2.3%20Views%20and%20Templates](http://www.grails.org/doc/latest/guide/6.%20The%20Web%20Layer.html#6.2.3%20Views%20and%20Templates) for some doc on this.

In a more advanced version, IntelliJ could probably also take care of the other attributes of the render tag.

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*Imported from [IDEA-66286](https://youtrack.jetbrains.com/issue/IDEA-66286) · Type: Feature · Votes: 6*
*Comments, attachments, dates and reporter are not part of the export — follow the link above for the full history.*

Start at the GSP editor's refactoring entry point and read the linked Grails Views and Templates documentation. Verify the flow for naming a template, creating it with the selected snippet, replacing the selection with a g:render tag, and handling the specified template name; the issue does not identify files or tests.

Written by the indexing model from the issue text.