Accessibility: Effects dialog doesn’t get focus

When reading a media file, open the Effects dialog (e.g. with Ctrl-E).
The dialog opens but the main window retains the focus, and there is
actually no way to access that dialog without the pointer (it doesn’t
appear in Alt-Tab under any of the *nix WMs I tested).

This is particularly annoying, as some of these Effects are particularly
useful for visually- or hearing-impaired users (for whom keyboard navigation
is absolutely necessary).

Reproduced under Debian and Fedora, both with VLC 3.0.12 and 4.0 nightly (dev 15525).

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