[2.3] Media browser tree icons

Converted from .png to FontAwesome icons.

Also increased size from 14px to 18px so they match the resource, elements tree icons etc.

Before:
![mb_before](https://cloud.githubusercontent.com/assets/825106/3583420/7252ec80-0c09-11e4-900a-6bbc1a3c2678.png)

After:
![mb_after](https://cloud.githubusercontent.com/assets/825106/3583423/77628bd6-0c09-11e4-9b1f-82ed2a162d64.png)