qt_add_translations with source autodetection mishandles id-based generated UI headers

Steps to reproduce:

1. Create a simple C++ GUI application with a single UI file
2. Ensure the UI file has the `idbasedtr="true"` attribute and is fully ID-based
3. Ensure the C++ uses `qtTrId` consistently
4. set `CMAKE_AUTOUIC_OPTIONS` to `-idbased` in `CMakeLists.txt`
5. Call `qt_add_translations` on the application target
   1. Without specifying `SOURCES` manually
   2. With `LRELEASE_OPTIONS -idbased`
6. Build the application
7. Run `update_translations`

This will produce a `.ts` file that contains translations with a context with an empty name, and which have the identifier as the source text, despite that the `ui_*.h` files use `qtTrId`, which should mark them as ID-based anyway. These duplicate the (correct) entries generated from the `.ui` files themselves.

I tried to avoid this in a couple ways besides specifying SOURCES manually, but none of them worked:

• Passing an extensions list that excluded .h files
• Using clang via a compilation database, with a build dir outside the source dir, and passing `-project-roots` to exclude it

This is because CMake generates a project JSON file that explicitly lists the generated headers, and that overrides both of these mechanisms as far as I can tell.

This seems to be a confluence of no less than three suboptimal behaviors:

1. `uic` fails to propagate the source text and comment fields to the header
2. `lupdate` fails to properly treat `qtTrId` in such headers (lacking a `//%` comment) as being ID-based
3. Despite `lupdate` supporting UI files directly, it's still passed the generated `ui_*.h` files as well

(apologies for the mangled early submission, was fighting with the rich text editor)