Document memory for Flatbooklet Snaplet

Signed-off-by: Heikki Salo <heikki.ao.salo@iki.fi>
Extracted Snaplet parts under src/Snaplet/

Signed-off-by: Heikki Salo <heikki.ao.salo@iki.fi>
Use STM for keeping repository state

Signed-off-by: Heikki Salo <heikki.ao.salo@iki.fi>