ImageViewer: Add up-to-date mimetypes (and add webp mimetype)

The list of mimetypes was missing new types like tga, dds and webp.
Just ask QImageReader::supportedMimeTypes() for the supported types,
which is also easier to maintain.

Task-number: QTBUG-45061
Change-Id: I80573da868088915b8d746c56374786e5e36903c
Reviewed-by: hjk <hjk@theqtcompany.com>
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>