add content icon to relateditems pattern

[petschki]

see #442

[vangheem]

How does this work OOTB? Does plone give a meaningful value for getIcon all the time. Should those img tags be conditionally in the template?

[davisagli]

It only returns a value for types that have an icon_expr set in portal_types. Most of the core types don't have an icon there (they get icons via CSS) but some addons still do. In plone.app.contenttypes file and image have a special getIcon indexer which gets an icon from the mimetypes_registry based on the file mimetype

[petschki]

So this is what the latest change brings OOTB:

[coveralls]

Coverage remained the same at 89.2% when pulling 7875a3e on issue-442 into 0478aff on master.

[vangheem]

I'm okay with this. This way you can have customized icons thumbnails and icons are the fallback.

Side note: We should match the icons used in Plone 5(defined as only css rules) back into the filesystem icon image files defined.