[CMAKE] Move .h files transformed from .ui to the form subdirectory

Summary:
This diff is an alternative to D2750.

This is a prerequisite for PR11651, as all the qt headers will be
included with their path relative to src/.

Example: #include "ui_intro.h will become `#include
<qt/forms/ui_intro.h>`.

The ui_*.h files generated from the *.ui files are generated by
qt5_wrap_ui in the CMakeLists.txt file directory, wherever the
source *.ui files are located.

This diff updates then restores the CMAKE_CURRENT_BINARY_DIR in order
to make CMake behave as if the CMakeLists.txt file was located into
the qt/forms directory.

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/D2786