Handle "conflicted" transactions properly

- Extend CMerkleTx::GetDepthInMainChain with the concept of a "conflicted" transaction - a transaction generated by the wallet that is not in the main chain or in the mempool, and, therefore, will likely never be confirmed.

- Exclamation mark icon for conflicted transactions
- Show mouseover status for conflicted transactions as "conflicted"
- Don't show inactive transactions on overview page overview