Make template more flexible

Fixed wrong navbar entries, document sectioning for readers and "thank you" big image clipping.

Utilize the \clip to constrain the image to the big circle. This leads to us being able to utilize bigger rectangular images, where we do not care for the corners.
I opted for a slight border between drawn backdrop circle (radius 38mm) and the image circle (radius 37.5mm) as having the exact same size lead to there being interpolation issues on the edges when utilizing the entire circle for an image, which looked bad.

The sections did not change anything with regards to the navigation balls, as they are controlled by the frame enviroments, not the subsections (It mearly shows some of the nav balls in a slightly highlighted color if there are multiple slides in a subsection).
This plentiful use of sections and subsections however lead to empty entries being visible in the generated pdf structure. By just removing them we regain a correct content display in pdf readers.

In the current implementation the intermediate slides will obtain a navigation dot within the navigation bar. By disabeling the miniframes for these slides we remove those unnecessary entries.

This prevents "jumping" of the author names in the center of the
footer due to the frame number needing more space in the next
frame (e.g., 9 --> 10 or 99 --> 100).

The default is 15%, which may (or may not) be too dark, so that
participants get distracted too easily. However, the transparency
should not be too light, as it serves as a hint so that listeners
see that the slide isn't fully uncovered. 8% seems to be a good
compromise. Of course, this depends on the projector...

Don't add vertical space if the subtitle or the date is missing
(= set to empty string).

* Add titlePageSpacing option to reduce the spacing on
  crowded title pages.
* Add titlePageNumberOfAuthors option if there are multiple
  co-authors which don't fit in the author bubble.
* Add institute to title page. Don't set \institute or do
  \institute{} to omit the institute.

This should make the size of the arrowhead independent of the
line size.