Bug report/feature request: allow fallback to normal estimate when M73 codes not present

[Patronics]

It seems with a recent update to Slic3r PE the M73 codes were disabled by default. Before I realized this was the case, and reenabled it, all my prints were stuck with an estimate of "1 minute, 0 seconds" throughout the entire print, rendering the eta estimate entirely nonfunctional. A much better behavior would be to automatically fallback to the conventional estimate when no M73 codes are present in the gcode.

[kazoob]

Also if you happen to start a print from the printer's SD card, the ETA will also be stuck at 1 minute since the plugin won't receive the M73 gcodes.