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.
- things that can be done in the designer file should be there,
not weirdly repeated in code elsewhere
- drop the insertion of an extra spacer (why not include it in the
designer file?)
- shuffle all the connect() calls down to the end of the constructor
- the "Select language" tooltip was applied to the form, so it would
show up inappropriately all over the place
- the buttons didn't have useful tooltips.
