The .ui files are all GPL-3.0-or-later style, but it's
slightly difficult to keep licensing information in them:
it's XML, so an XML comment might work, but there's no
guarantee that safe/load will preserve them.
Put the SPDX tags in the <author> tag, so that it's visible
in Qt Designer.
Document ChoicePage because it's quite long and confusing.
Add PCM state preview and update it when a choice is selected.
Smaller icons.
Update strings.
