add styled scrollbars

Hallo, I improved the scrollbars for the tree view and the basic editor:

![scrollbars](https://cloud.githubusercontent.com/assets/670641/4844479/a54e65d8-603c-11e4-80e9-61efedcc1072.png)

There is a minimal difference in colors between these views, so I adapted the color of selection. In addition I set the width for vertical scrollbars and height for horizontal scrollbars.

Feel free to merge. ;-)

Happy coding,
Darius