[CMAKE] Avoid displaying the console when launching bitcoin-qt.exe

Summary:
This adds a flag which prevents displaying the console when running
bitcoin-qt.exe.

Test Plan:
Prerequisite:
Follow the doc/build-windows.md and depends/README.md documentation
and install the dependencies for i686-w64-mingw32 and
x86_64-w64-mingw32.

# 32-bit version
mkdir buildcmake && cd buildcmake
cmake -GNinja .. -DBUILD_BITCOIN_SEEDER=OFF \
  -DCMAKE_TOOLCHAIN_FILE=../cmake/platforms/Win32.cmake
ninja

Check the build succeeds.
Check the generated src/qt/bitcoin-qt.exe binary runs as expected.
Ensure there is no console display when running the exe.

Operate the same for Win64.

# 64-bit version
rm -rf *
cmake -GNinja .. -DBUILD_BITCOIN_SEEDER=OFF \
  -DCMAKE_TOOLCHAIN_FILE=../cmake/platforms/Win64.cmake
ninja

Check the build succeeds.
Check the generated src/qt/bitcoin-qt.exe binary runs as expected.
Ensure there is no console display when running the exe.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2682