Bitcoin-Qt: full Qt5 support

- re-work included headers and used functions to be fully Qt5 compliant,
  while maintaining Qt4 compatibility
- also includes some small GUI layout changes (remove hard-coded column
  sizes)

Below is a screen with the Qt5 Fusion style :).
![Qt5 Fusion](https://f.cloud.github.com/assets/1419649/340904/d30a46a0-9d5f-11e2-9377-a5976621372e.png)