Prevent overwriting doc images across projects

QTBUG-129444 shows an issue where two different projects in two different submodules used the same filename for two different doc images, causing one to overwrite the other when generating HTML output.

It is not unlikely that the same filename is used by independent projects. Guaranteeing unique filenames across all of Qt without tooling support is challenging. It would therefore be very useful if QDoc did not require unique filenames, or gave warnings that would be caught by some of our infrastructure.

Two not so elegant way of doing this would be to combine the filename with a hash over the file content, or encode the full image path into the filename when copying to the destination folder. I assume there can be better ways of solving this.