Suggested by Caolán McNamara <caolanm@redhat.com>.

Additionally, fixes fdo#30667 with new about dialog text, and a
link to http://libreoffice.org/credits.html

Added a svt::FixedHyperlink to the about dialog, so people can
click on something to get to the credits page.

As per ODF 1.2 Part 3, 4.8.10:

A manifest:media-type attribute should be present for all files and
directories where a MIME media type exists for the content of the
file...

So added "image/png" for the thumbnails there.

Otherwise there are two checkboxes, and since it's difficult to get
at the KFileDialog one, just assume it does the right thing (which
additionally makes it consistent with other KDE apps).

Removing compile time warnings about enum values not being handled in select statement.