fix tests
Filter out references that are None.

This may happen when the reference_catalog has links from a source to
a no longer existing target.
Nuke trailing white space.
pep8
Fix test for contents of base widget html.
Replace deprecated test assert statements.
Do not use portal_interface tool but @@plone_interface_info (PLIP #13770)
fixing view invocation
Merge pull request #14 from plone/plip-13770-portal-tools-removal

PLIP 13770 remove portal_interface
Bump version.
Preparing release 2.5.0
Back to development: 2.5.1
Merge branch 'master' into bugfix-13853-selected-items-not-checked