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 diffs implements a modified version of the `qt5_wrap_ui` function
which allows to set the destination directory of the generated files.