fix long definition not shown properly in dictionary window

Now long definition in dictionary window is scrollable with swipe gesture.

This should fix bug reported in #132.

![screenshot from 2013-07-28 16 11 16](https://f.cloud.github.com/assets/751535/867971/66824a8e-f75d-11e2-8304-deb478d2a7c8.png)