TinyMCE: Lost ability to choose which content "Contains objects"

[frapell]

Seems like back in 2017 (!) c083075 we inadvertently broke the ability to use the "Contains objects" setting from the "Resource Types" tab in the TinyMCE control panel.

This setting allows you to control which content items are folderish, and so allow the relatedItems widget to navigate inside.

Based on the tags where the commit was included, this affects Plone versions 5.1.0+, I will be submitting PR's for 5.1, 5.2, 6.0 and 6.1 versions

[frapell]

@petschki @MrTango @thet I submitted PR for all affected versions, included master, if one of you could give it a quick review please :)

[vamshi567bh]

@frapell I would like to this

[stevepiercy]

@BharathC0 please read and follow First-time contributors, especially Things not to do, and Contributing to Plone.