Make footer element4 also changeable, for institute logos

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.
1 job for feature--logo-jsc