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.
We get the mountpoints already used by other partitions, and
disable the Ok button in the "Create new partition" dialog if
the user selects/writes a mountpoint which is already used.
We are going to do the same in the Edit partition dialog
after testing.
