qt: Add option to (not) spend unconfirmed change

- Add a wallet tab to options dialog
- Move fee setting to wallet tab
- Add new setting to set -spendzeroconfchange from UI

![spend-unconfirmed-gui](https://f.cloud.github.com/assets/126646/2177543/bae02288-9625-11e3-8ddc-a17f3d74065d.png)

Implementation is straightforward, messages are up for discussion as usual.