qtui: Make the debug log a proper dialog

For some reason unbeknownst and shrouded in ancient mystery, the
debug log was displayed in a naked widget, which caused it to not
be rendered in a window when using not-so-smart window managers
(such as Windows™).

Make the debug log a proper dialog, and also fix the close button
not being part of a platform-agnostic QDialogButtonBox, while we're
at it.