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)

What's the reason for making this an extendable definition? Can't it be inlined in the mixin? Sorry, not too versed in SASS