[CMAKE] Complete the dist directory to prepare for the DMG image build

Summary:
This diff adds the remaining files to the dist DMG folder in order to
later generate the DMG image from it.

The remaining files are:

• The .DS_Store file (Desktop Services Store). This one is generated

from a python script from the contrib/macdeploy directory.

• A symlink to the /Applications folder to allow for drag'n drop

install.

Depends on D4381.

Test Plan:

ninja osx-deploydir

Check the dist directory contains the expected files.
Note: generating the .DS_Store using the python script will require
some dependencies if not using the (coming) gitian build. These
dependencies can be installed with:

pip3 install biplist ds_store mac_alias

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4383