[fix] Restore PAGE_MARGIN string (#4993)
[fix] Restore PAGE_MARGIN string (#4993)

It's used by KOptConfig

Fix #4992
Regression since #4945
NiLuJe/koreader,poire-z/koreader,koreader/koreader,Frenzie/koreader,NiLuJe/koreader,Markismus/koreader,pazos/koreader,Frenzie/koreader,mihailim/koreader,poire-z/koreader,Hzj-jie/koreader,koreader/koreader,mwoz123/koreader