WebUI: Use setProperty() for selecting an option

[JimmyAx]

This pull request resolves an issue with Safari not properly selecting an <option>. Unlike other browsers Safari does select an <option> when the selected attribute is set using setAttribute(). Using setProperty() solves this issue.

I have tested it by patching qBittorrent version 4.5.2. That means I have not tested the log view as it is not present in that version. Tested with Safari version 16.4 on macOS. I have also verified that it is still working in Firefox 113.0.2.

Closes #17866.