Xournalpp Collaboration Mode
This is a first attempt for a collaboration mode for Xournalpp.
Each user has its own file, and selects the other's file as "Collaboration file".
Then, as soon as some user saves its own file, the other instance automatically detects the change and updates it.
More precisely, each user should modify only their "Layer 1", while "Layer 2" is used for importing the other user's modifications.