[Qt] Add column Watch-only to transactions list

- Changes the condition for showing watch-only to if there has been watch-only imported or not, instead of looking at the balance. The labels should be shown, even with a balance of zero, if you run importaddress.
- Adds new filterable dynamic column Watch-only

![watchonly_column4](https://cloud.githubusercontent.com/assets/2814559/3869705/126a03c2-20a3-11e4-9685-2e5a0e05250f.png)

![watchonly_column1](https://cloud.githubusercontent.com/assets/2814559/3869701/099cd6fc-20a3-11e4-8c5e-0e673c7fba3f.png)

![watchonly_column2](https://cloud.githubusercontent.com/assets/2814559/3869702/0e8174ac-20a3-11e4-9ae7-b35947d42ebe.png)