Experimental Release: v1.2.3 + relative PDF paths

This is a patch based on the 1.2.3 release.
The 3 commits from PR xournalpp#4262 have been applied by cherry-picking.

Experimental Release

Experimental release from "relative3" branch

v1.2.3+relative+jkg

v1.2.3 + cherry-picked commits + specific settings for school usage