[Qt] some cleanup and watch-only changes to transactiontablemodel

See new color and better recognizable watch-only tag:
![watch-only](https://cloud.githubusercontent.com/assets/1419649/3866301/70b5c49a-1fca-11e4-80c6-32eeea549bb9.png)
@laanwj I guess this could just ``return QVariant();`` and I could remove COLOR_BLACK?