Add extra preview-mode selector to the CMS actions so we can show
something when the preview is closed (and with it all options are not
visible).
Thanks @mateusz, @clarkepaul and @robert-h-curry for contributing.
1. Modified gridfield to use default colours
2. Added ability to show and hide the filter
3. Design changes to icons used
4. Gridfield template fixes: Took out broken mark-up from templates
(spans were being inserted inside spans), and adjusted styles. Added
extra class to template
5. Added "filter by ..." placeholder text to input fields