Wallet/transaction notifications: Only a single notification is displayed

[luke-jr]

There are actually multiple issues here, at least:

1. WalletView::processNewTransaction only looks at the first of a batch of inserted rows.
2. QSystemTrayIcon's X11 implementation (QBalloonTip, internal-only) only allows a single notification, destroying the previous when a new one is shown
3. Showing possibly hundreds of notifications would be terrible UX

It seems like the GUI should queue notifications at the same "instant" (eg, a new block), and if there's multiple send a summary of them all (number of transactions, total sent, total received/mined). This may require some refactoring work, as notifications currently follow the TransactionTableModel insertion slots.