qt: Rename 'Confirmed' balance to 'Spendable' on overview page

The word 'Spendable' more precisely says what the balance actually means.

Avoids the confirmed/unconfirmed confusion that can be caused by #3676.
Merge pull request #3699

ddcabae qt: Rename 'Confirmed' balance to 'Spendable' on overview page (Wladimir J. van der Laan)