Smarter Completion for Grails Specific Files

IntelliJ could be smarter with the completion when editing Grails specific files:

- completing after `def scaffold = <>` in a controller should offer only classes that make sense: e.g. only domains.
- completing in the `static constraints = {}` section of a domain class should only offer the fields of the domain and it's parents. The same for `static mapping = {}` and other similar "convention" based Grails functionality.
- completing in UrlMappings should offer really only what's allowed to map there.

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*Imported from [IDEA-123542](https://youtrack.jetbrains.com/issue/IDEA-123542) · Type: Usability Problem · Votes: 0*
*Comments, attachments, dates and reporter are not part of the export — follow the link above for the full history.*

Start by locating the Grails IntelliJ completion handling for controllers, domain classes, UrlMappings, and convention sections such as constraints and mapping. Reproduce completion in each context and document the currently offered items; done means each context offers only the Grails-valid classes, fields, or mappings described in the issue.

Written by the indexing model from the issue text.