[Qt] optionally show debug window as main tab

I would find it more convenient, if the debug window could be shown as main tab.
Changing the option needs client restart.

![debugtab2](https://f.cloud.github.com/assets/2814559/2492095/ca027132-b211-11e3-9d2c-5b984699064f.png)

![debugtab1](https://f.cloud.github.com/assets/2814559/2492097/cdc927f2-b211-11e3-88e5-32b440667d59.png)