[CMAKE] Add QT plugins according to the target platform

Summary:
If the QT build is static, add the minimal integration plugin and any
platform specific plugin depending on the target.

Depends on D3091

Test Plan:

mkdir buildcmake && cd buildcmake
cmake -GNinja ..
ninja
./src/qt/bitcoin-qt

Check that bitcoin-qt runs.

rm -rf *
cmake -GNinja -DCMAKE_TOOLCHAIN_FILE=../cmake/platforms/Win64.cmake \

-DBUILD_BITCOIN_SEEDER=OFF ..

ninja

Run the generated bitcoin-qt.exe on a 64 bits Windows.

rm -rf *
cmake -GNinja -DCMAKE_TOOLCHAIN_FILE=../cmake/platforms/Win32.cmake \

-DBUILD_BITCOIN_SEEDER=OFF ..

ninja

Run the generated bitcoin-qt.exe on a 32 or 64 bits Windows.

Note: OSX build needs another fix to build.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3017