#1723 Add time to read into the status bar

Add new statuses:
TB - book time to read
TC - chapter time to read

Fix backward compatible in statistics plugin

plugins/statistics.koplugin/tableutil.lua

@@ -25,10 +25,4 @@ function tableutil.spairs(t, order)
     end
 end
 
-function tableutil.tablelength(T)
-    local count = 0
-    for _ in pairs(T) do count = count + 1 end
-    return count
-end
-
 return tableutil