gui: stop stacking every orphan the node ever produced on top of the list (#67)

The wallet list sorted on GetDepthInMainChain() ascending so pending
transactions came first. An orphaned coinbase is also depth 0 -- and it
stays depth 0 forever, because it is never going to be confirmed.

So every orphan a node had ever produced sat permanently at the top of the
list, above every block it had actually won. A miner with a week of
ordinary orphaned races opened the wallet and saw nothing but failures.
The balance was right the whole time; the list was telling a different
story. That is how this was found: a screenshot of a healthy,
correctly-mining node that looked like it was failing at everything.

Sort on (rank, depth) instead, where rank is 0 for pending, 1 for
confirmed, 2 for orphaned. Orphans go to the bottom, pending stays on top,
confirmed stays ordered most-recent-first.

Verified against a datadir whose chain had been reorganised away, so every
coinbase in it is orphaned: all 17 wallet transactions classify as rank 2.
Not tested against a mixed wallet -- the machine that has one is in
production with its datadir locked by the running node.

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