use the painter fontMetrics() to obtain the right text size

On my system when doing a Move Component Text (Ctrl+K) the text bounding box shown is not right. It turns out that one needs to use the painter fontMetrics() instead on relying on the current font metrics, not sure why. Might affect Linux only, did not check on other OSs.
![qucs_0 0 19_move_text](https://cloud.githubusercontent.com/assets/9018179/4875004/2eed8e80-627d-11e4-940b-7a3790b68335.png) ![qucs_0 0 19_patched_move_text](https://cloud.githubusercontent.com/assets/9018179/4875006/3490a1c4-627d-11e4-9874-b93d2894ae28.png)