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
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.