Remove unnecessary dependency on plone.app.relationfield

[alecghica]

Right now collective.cover depends on plone.app.relationfield which can easily made optional. This (unused?) dependency brings more problems than benefits and can be added independently if needed.

Problem is that plone.app.relationfield is dependent of plone.app.intid which, on install, loops through the whole catalog and does a getObject on each brain:

• https://github.com/plone/plone.app.intid/blob/master/plone/app/intid/setuphandlers.py#L24

... this operation will never succeed if you have a medium or large database already, with ten of thousand of objects/brains for example and you will never be able to install the collective.cover.

Removing dependency of plone.app.relationfield can be done easily by removing the followings:

• https://github.com/collective/collective.cover/blob/master/src/collective/cover/profiles/default/metadata.xml#L10
• https://github.com/collective/collective.cover/blob/master/src/collective/cover/profiles/default/types/collective.cover.content.xml#L25
• https://github.com/collective/collective.cover/blob/master/src/collective/cover/configure.zcml#L14

[hvelarde]

a PR is highly appreciated; plaase document what to do on upgrades to not break current installations, the same we did with other hard dependencies we have been removing in the last months.

[alecghica]

I won't have the time for this in the near future so anyone interested in adding a pull request for this is welcomed.