Adds "Better rendering of list items" to Style tweaks (#3968)

Alternate rendering of list items as block elements (instead of
inline/final).
Check the built DOM is still coherent with styles on stylesheet
change, and propose to reload the document if not. Also
invalidate cache on close in that case so a new DOM is built
at next opening.
ReaderStyleTweak: fix tweak css duplication; when globally
enabled, when disabling and re-enabling locally, the css would
be added twice to the final CSS.
Also adds !important to "sub_sup_smaller" tweak, and increase
its priority so it can override "font_size_all_inherit".
Adds CreDocument:getCacheFilePath() for futur features.

frontend/apps/reader/modules/readerstyletweak.lua

@@ -263,8 +263,10 @@ function ReaderStyleTweak:updateCssText(apply)
         local tweaks = {}
         for id, enabled in pairs(self.global_tweaks) do
             -- there are only enabled tweaks in global_tweaks, but we don't
-            -- use them if they are disabled in doc_tweaks
-            if self.doc_tweaks[id] ~= false then
+            -- add them here if they appear in doc_tweaks (if enabled in
+            -- doc_tweaks, they'll be added below; if disabled, they should
+            -- not be added)
+            if self.doc_tweaks[id] == nil then
                 table.insert(tweaks, self.tweaks_by_id[id])
             end
         end