qt: Modernize 'confirmed' terminology in shown tx status

These days we regard transactions with one confirmation to be
'Confirmed'.
Waiting for 6 confirmations is a recommendation but should not
keep the transaction shown as unconfirmed.

Misc code sanity:

• Merge maturity/status enums, they had become completely disjunct
• 'confirmed' flag is now called 'countsForBalance' for clarity