Changes to user manual

[Willem Ferguson]

I submitted a PR with a small C++ code change (dive list context menu)
that you already know about as well as the changes to the user manual to
reflect Berthold's work with videos. It took much more work than I had
anticipated.

There are about nine other images in the user manual that still show the
old Photos tab, and not he current Media tab. But most of them are not
simple cases of screenshots, but are images that require some graphic
manipulation. I will not get to that by the time Dirk wishes to publish
the new release. But ALL screenshots pertaining remotely to media are up
to date and, as far as I know, correct. I will work on the other images
within the next week. I did a spelling check of the text and it should
be ok.

Berthold, are you able at all to look at my PR and see if this looks
satisfactory to you at all? Feel absolutely free to modify. I put the
technical details of the video thumbnails in an appendix that is
hyperlinked to the main text dealing with media.

I have a mail from Dirk saying that he will deal with the issue of
appdata.xml that is inadvertently changed by the build script.

Kind regards,

willem