[CMAKE] Build the OSX application bundle for bitcoin qt

Summary:
When building/cross building for OSX, create an application bundle that
can run natively on OSX.
This is a first step toward generating an installer for OSX.

Test Plan:
On OSX (or cross build):

ninja BitcoinABC-Qt

Check there is an app bundle (BitcoinABC-Qt.app) created under
src/qt.
Check on OSX that the bundle has the correct icon, details and can be
run.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4290