Devoncarew fileview markers

Display error and warning markers in the files view. Listen for changes to the markers and update the display appropriately.

- changes to the files view and related code to be able to get a FileItemCell given the corresponding Resource
- markers are displayed using `:after` css selectors. Using `div`s would have required a lot of changes to the layout code for file items. We may want to look into using flex layout for file item rendering in the future
- changed the sorting for the files view to sort loose top-level files before top-level folders
- some minor API changes to markers support showing them in the files view

@dinhviethoa 

![screen shot 2014-01-05 at 11 28 37 am](https://f.cloud.github.com/assets/1269969/1852159/f7fe7bf0-76f5-11e3-9912-f8dfef7b4619.png)