expression builder: add splitters and change buttons

This adds two sliders to the dialog: 

* one vertical between function list and selected function help
* one between the top and the expression part (to have more lines in case of long expressions)

Also, I rephrased the buttons "load values". OK for the buttons?

![image](https://f.cloud.github.com/assets/127259/1910611/c793f25c-7d1b-11e3-85a1-49632f5de270.png)