[CMAKE] Move-only: refactor qt/CMakeLists.txt

Summary:
This is a move only diff that puts related things together.
It tries to avoid duplicating the tests in the file, and moves the
static build section after the bitcoin-qt-base target is defined
(prepare for adding the platform plugins to the target).

Test Plan:

mkdir buildcmake && cd buildcmake
cmake -GNinja ..
ninja check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3091