Hide non-linear fragments

Add option to hide (skip) non-linear fragments, only working
in 1-page mode. Tweaks mostly to footer, toc and skim code
to make it clear(er) which pages belong to linear or non-linear
fragments.

frontend/ui/elements/reader_menu_order.lua

@@ -16,6 +16,7 @@ local order = {
         "toggle_bookmark",
         "bookmark_browsing_mode",
         "page_map",
+        "hide_nonlinear_flows",
         "----------------------------",
         "go_to",
         "skim_to",