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)

Hey @chillu, the idea of using an extend is actually to prevent duplicated CSS. `@extend` rules are only written once and any applicable selectors are collected and applied to the one rule.

If I included all of that CSS in the mixin it would be duplicated every time I used the mixin. So I put all the re-usable stuff in the extend, and put the variables in the mixin.

You can see the result here: https://github.com/jonom/silverstripe-framework/blob/sitetree-status-icons/admin/css/screen.css#L851