[qt] Add use available balance in send coins dialog

Summary: Backport of Core PR #11316. (Commit d052e38)

Test Plan:

make VERBOSE=1 check

Then test the gui.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix, jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1699