[CMAKE] Build the OSX application bundle for bitcoin qt
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.
On OSX (or cross build):
Check there is an app bundle (BitcoinABC-Qt.app) created under
Check on OSX that the bundle has the correct icon, details and can be
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D4290