Statistics: Add percentage completed to Statistics plugin

Makes percentage completed easily accessible through the statistics plugin. It has always been accessible through the status bar, but not through the statistics menu.

plugins/statistics.koplugin/main.lua

@@ -310,6 +310,7 @@ function ReaderStatistics:getCurrentStat()
         { _("Total days"), total_days },
         { _("Average time per page"), util.secondsToClock(avg_time_per_page, false) },
         { _("Read pages/Total pages"), read_pages .. "/" .. self.data.pages },
+        { _("Percentage completed"), math.floor(read_pages / self.data.pages * 100 + 0.5) .. "%" }, -- adding 0.5 rounds to nearest integer with math.floor
     }
 end