Support custom Artefact types/ordering in Grails Tool view

Some Grails plugins create new Artefact types; e.g., the [JAXRS](https://github.com/krasserm/grails-jaxrs/wiki) plugin creates new Resource and Provider types. But, these new Artefacts are buried in the middle of the explorer tree (under grails-app/resource etc) rather than grouped and highlighted like Domain, Controller, Services, and similar existing Artefacts. The Grails view should allow for promoting those any declared Artefact to be at the same level in the explorer tree as the existing Artefacts. (Actually, it should be more general - the top-level entries in the explorer view should be drag-droppable to suit the user's current preferences, rather than having Taglibs be at the bottom ...)

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*Imported from [IDEA-135245](https://youtrack.jetbrains.com/issue/IDEA-135245) · 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 Tool view and its explorer-tree implementation, then trace how existing artefact types are grouped and ordered. Define how declared custom artefacts and user-controlled top-level ordering should be represented; done means custom types can be promoted and the displayed order follows the user's preferences.

Written by the indexing model from the issue text.