Add a lint indicator for Interactive Linter linting

Removes default Brackets lint status indicator.
Replaces the `errorIndicator` with this `lintIndicator`.

See #61 for more detail on the reasons for this. 
Another magic number, but in this case I (may) have a solution in [`AnimationUtils`](https://github.com/adobe/brackets/blob/master/src/utils/AnimationUtils.js).