WIP: Remove transaction proposals referencing already spent inputs

This PR adds methods to the wallet object to remove pending TXPs which reference already spent inputs. Integration with the UI is still a work in progress.

Fixes #1170

The idea is to call this method from updateTx (called from the Send and History tabs). The problem is that updateBalance is not always called before this method. This means I would have to cache the unspent list whenever updateBalance is called and use this cached value from within updateTx. Does this sound like an acceptable solution?