Refactored editors and imageviewer (fixed #348)

- Improved tabs visual style
- Refactored editor manager and image viewer
Fixed #348

@devoncarew 

screenshot:
![screen shot 2014-01-24 at 3 32 22 pm](https://f.cloud.github.com/assets/332168/2000039/d9589d44-854f-11e3-8753-6f31725edc88.png)

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