cre: toggable enhanced rendering (floats, collapsed margins...)

Adds "Render mode" toggle with 4 modes, to enable some or all
of crengine's new enhanced block rendering features.
Use "legacy" rendering for previously opened books to not mess
with bookmarks, and "web" (all of the new features) for new books.

Also make available the added "HTML5" stylesheet to the Style menu.

frontend/apps/reader/modules/readerbookmark.lua

@@ -586,4 +586,8 @@ function ReaderBookmark:getLatestBookmark()
     return latest_bookmark
 end
 
+function ReaderBookmark:hasBookmarks()
+    return self.bookmarks and #self.bookmarks > 0
+end
+
 return ReaderBookmark