Set Dialog to look more like Brackets

I really like the idea of this extension and found it already very helpful.

The settings dialog looked alright with the iCheck checkboxes, but it didn't really look like Brackets.
With this pull request I tried to change that and make the design closer to original Brackets (highly inspired by the extension dialog).
![php_smart_hints - pr](https://cloud.githubusercontent.com/assets/2564094/4120552/02ab4cb2-32b3-11e4-9c18-91caa7fd46e0.png)

I have also replaced the two buttons with a [tri-state-checkbox] (http://css-tricks.com/indeterminate-checkboxes/) that can be used to select or deselect all, but also shows the state of the current selection.

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Please let me know what you think of this, as I am not the design expert - just sometimes a bit picky. :wink: