[CMAKE] Create a windows installer
[CMAKE] Create a windows installer

Summary:
This diff adds NSIS to the windows package generators, and configure it
to create a windows graphical installer/uninstaller.

The generated installer is very similar to the one from autotools, the
main difference is that this installer asks for licence agreement.

The installed files differ from autotools, which installs a lot of files
in a custom subtree, including documentation on how to build the
software and a lot of things which are unexpected output from a
graphical installer. This one only installs the files from the package,
identical to what is distributed as part of a release.

Depends on D4284.

Test Plan:
  mkdir buildcmake && cd buildcmake
  cmake -GNinja .. -DBUILD_BITCOIN_SEEDER=OFF \
    -DCMAKE_TOOLCHAIN_FILE=../cmake/platforms/Win64.cmake
  ninja package
Check a `bitcoin-abc-<version>-x86_64-w64-mingw32.exe` installer is
created.
Run the installer, check all the steps are correct.
After installation run `Bitcoin ABC` and `Bitcoin ABC (testnet)` from
the start menu folder.
Uninstall the software using then windows settings panel and check all
the files are deleted.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4274

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