Always update the table of footer text generators (#6563)
Always update the table of footer text generators (#6563)

It's only needed by all_at_once, but we were skipping it if nothing or
only a single mode was enabled.
It lead to desync w/ the settings, or a crash.

Fix #6561
NiLuJe/koreader,Hzj-jie/koreader,Markismus/koreader,pazos/koreader,mwoz123/koreader,poire-z/koreader,koreader/koreader,Frenzie/koreader,poire-z/koreader,koreader/koreader,Frenzie/koreader,NiLuJe/koreader