[fix] Restore PAGE_MARGIN string (#4993)

It's used by KOptConfig

Fix #4992
Regression since #4945

frontend/ui/data/strings.lua

@@ -8,6 +8,7 @@ S.PAGE_CROP = _("Page Crop")
 S.FULL_SCREEN = _("Full Screen")
 S.SCROLL_MODE = _("Scroll Mode")
 S.ZOOM_DPI = _("Zoom (dpi)")
+S.PAGE_MARGIN = _("Margin")
 S.H_PAGE_MARGINS = _("L/R Margins")
 S.T_PAGE_MARGIN = _("Top Margin")
 S.B_PAGE_MARGIN = _("Bottom Margin")