increase scrollbar repeat buttons and thumb

changes for #1033 

before
![2014-02-22_13h13_05](https://f.cloud.github.com/assets/658431/2237665/9d9a848e-9bbb-11e3-8f27-48db8ed99c47.png)

after
![2014-02-22_13h14_53](https://f.cloud.github.com/assets/658431/2237666/b0bb6790-9bbb-11e3-8507-8e9a24cfac5e.png)