Font menu: add symbols for default and fallback fonts (#3941)

Also allows for updating the fallback font and see results
in real-time on the underlying document.
Bump crengine for: fix updating fallback font

Also adds a menu item to generate a html document showing some
sample text rendered with each available font (only if the user has
a file koreader/settings/fonts-test-sample.html with some HTML
snippet of his choice).

frontend/ui/widget/touchmenu.lua

@@ -726,8 +726,12 @@ function TouchMenu:onMenuHold(item)
         end
         if callback then
             UIManager:scheduleIn(0.1, function()
-                self:closeMenu()
-                callback()
+                if item.hold_may_update_menu then
+                    callback(function() self:updateItems() end)
+                else
+                    self:closeMenu()
+                    callback()
+                end
             end)
         end
     end