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

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