[Feature request] Printing time

[luxarts]

Hello!

It's very useful to see the current print time in the main screen. As the printed time is already stored all you have to do it's at the begining of the print store that value in a new variable and show the difference between both values (current time - start time). I added it very roughly because I'm modified the firmare to adapt my hardware but was easy to archieve and doesn't affect the calculation time, print speed or anything similar (at least in 32 bit cpu).

Regards, Lucas