Add values_order key to getVocabularyValues for retaining manual sorting

[JeffersonBledsoe]

Adds a values_order key to the return of getVocabularyValues so that we can un-sort the titles should we want to.

Fixes #161 without breaking the existing sorting

[mister-roboto]

@JeffersonBledsoe thanks for creating this Pull Request and helping to improve Plone!

TL;DR: Finish pushing changes, pass all other checks, then paste a comment:

@jenkins-plone-org please run jobs

To ensure that these changes do not break other parts of Plone, the Plone test suite matrix needs to pass, but it takes 30-60 min. Other CI checks are usually much faster and the Plone Jenkins resources are limited, so when done pushing changes and all other checks pass either start all Jenkins PR jobs yourself, or simply add the comment above in this PR to start all the jobs automatically.

Happy hacking!

[davisagli]

Instead of returning 2 different structures (one with the original order and one sorted by title) would it make sense to support vocabularies that specify their own custom sort function as an attribute? (which could be a no-op)