Dispatcher: avoid multiple cre rerenderings when many settings changed (#9651)

When a gesture/profile was updating multiple cre settings,
each setting handler would emit UpdatePos which each would
force a re-rendering.
When this might be happening, postpone the rerendering
until all are set.
Needs some bit of refactoring to the events at play:
introduce "DocumentRerendered" event, and use it where
we used "UpdatePos" or "UpdateToc" to mean exactly that.

frontend/apps/reader/modules/readertoc.lua

@@ -99,11 +99,12 @@ end
 function ReaderToc:onUpdateToc()
     self:resetToc()
     self.ui:handleEvent(Event:new("TocReset"))
-
-    --- @note: Let this propagate, plugins/statistics uses it to react to changes in document pagination
-    --return true
+    return true
 end
 
+-- Be sure to update the ToC after a CRE rerendering
+ReaderToc.onDocumentRerendered = ReaderToc.onUpdateToc
+
 function ReaderToc:onPageUpdate(pageno)
     if UIManager.FULL_REFRESH_COUNT == -1 or G_reader_settings:isTrue("refresh_on_chapter_boundaries") then
         local flash_on_second = G_reader_settings:nilOrFalse("no_refresh_on_second_chapter_page")