Table: Columntoggle: Take into account <abbr> title when generating columntoggle checkbox. Fixes #6053 - abbr title in th not picked up for column toggle.
Unit tests: Columntoggle table: Make sure the presence of an <abbr> with a title attribute in the <th> overrides the <th> text in the checkbox (#6053).
Gruntfile.js: Update filter for non-existing files to use OS-agnostic path-handling.