CoverBrowser: fix files_per_page setting type (#5556)
CoverBrowser: fix files_per_page setting type (#5556)

Avoid saving/getting "12.0" (as the lua-ljsqlite3 driver does),
and save/get "12" instead by properly converting it to a string
and back, as the config.value table column is of type TEXT.
pazos/koreader,mwoz123/koreader,NiLuJe/koreader,poire-z/koreader,Hzj-jie/koreader,koreader/koreader,NiLuJe/koreader,Markismus/koreader,Frenzie/koreader,poire-z/koreader,Frenzie/koreader,mihailim/koreader,koreader/koreader