qt: Rename 'Confirmed' balance to 'Spendable' on overview page

The word 'Spendable' more precisely says what the balance actually means. It is not always 'confirmed' (for example in the case of spending unconfirmed change #3676).

![scr2](https://f.cloud.github.com/assets/126646/2195082/06fd9006-9897-11e3-9c72-67b220ce7e86.png)