Finally key shortcuts:

'U' -> Undo
'R' -> Redo
CTRL-S -> Save As
CTRL-N -> New
CTRL-O -> Open (also for Annotating PDF)
CTRL-Q -> Quit (asks if unsaved) [probably a bug, should behave as when closing it with the X powerkey]

If you tap the X icon to close Xournal, it will autosave your work in a temporary file and will reload the temporary file when re-launching it.

I plan to leave +/- for Volume settings and use keys to zoom in/out.
What do you guys think?