AUTOUIC: Collect ui header files for Ninja generator
The '_autogen/timestamp' target supposed to generate ui header files using the 'uic'. Ninja must have information about these header files as a result of generating.
The fix collects .ui files of the user target and generates a list of the ui headers that need to be added to the generating results of the '_autogen/timestamp' target.
The case when the .ui files are not specified and collected by AUTOUIC from the include directives of the project source files is not covered in this patch.
Fixes: #16776
Topic-rename: autouic-headers-ninja