NEW: CMS site tree status icons

Added coloured status 'dots' to overlay page icons and improved visibility of status labels. As discussed in https://github.com/silverstripe/silverstripe-framework/pull/2794

![img](https://cloud.githubusercontent.com/assets/1079425/5371903/366f3704-8090-11e4-9481-16d9e81a0684.png)

That'll create a lot of duplication in the generated CSS, can you please refactor to use standard CSS classes for the `%tree-status-icon-before` bit?