[JENKINS-20739] Filter descriptors before adding them to hetero-list.

This allows implementations of DescriptorVisibilityFilter to hide items
with descriptors from users to prevent them from being added via the UI.

This is already implemented in /lib/hudson/newFromList/form.jelly, but
nowhere else.
Added logging when filtering descriptors.
Merge pull request #1033 from daniel-beck/filter-descriptors

[JENKINS-20739] Filter descriptors before adding them to hetero-list