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 diffs move the translation to a new CMakeLists.txt located in the
qt/forms subdirectory, so the ui_*.h files are generated here.