THIS MERGE PROBABLY NEEDS TO BE REBASED AFTER !13 (merged) IS ACCEPT, AS IT BUILDS UP ON IT. MESSAGE ME. I WILL DO IT! (I just had a few minutes to spare and wanted to get this one here out of my head.)
This implements one leftover merge request from a year ago, !6 (closed), and generalizes it on the way.
Just like for footer element{1..3}, footer element4 can now be set via \setbeamertemplate. This will replace the FZJ logo in the lower right corner by the user's choosing. Intended usage is a FZJ logo which has the institute name to the right of it.
That also is the main difference of \setbeamertemplate{footer element4} to 1..3: The vertical margins are adapted to match the align of the FZJ logo.
A section has been added to the tutorial to show and explain the usage.