cre landscape view: adds 1 page / 2 pages toggable

Adds a toggle switch in the Orientation bottom config menu to
allow showing 1 page or 2 pages when in landscape mode.
Previously, this was hardcoded to be in 2-pages modes only
in some circumstances (device resolution + user dpi).

frontend/ui/data/strings.lua

@@ -3,6 +3,7 @@ local _ = require("gettext")
 local S = {}
 
 S.SCREEN_MODE = _("Orientation")
+S.TWO_PAGES = _("Two Pages")
 S.PAGE_CROP = _("Page Crop")
 S.FULL_SCREEN = _("Full Screen")
 S.SCROLL_MODE = _("Scroll Mode")