Fix the width of the bubble tip

If you define a tooltip and then resize the window the tooltip won't work as expected.

Sample width no fixed size:
![no_fixed_width](https://f.cloud.github.com/assets/5265188/1109324/2930aa48-197e-11e3-8bed-6584fd1b664c.png)

Same width fixed size:
![fixed_width](https://f.cloud.github.com/assets/5265188/1109325/29518146-197e-11e3-83b1-9efab096271f.png)

Maybe there is a lovely way to solve it but this one was easy.