Doesn't handle Slic3r gcode

[ctgreybeard]

Two initial problems with trying to process gcode output from Slic3r:

1. The G21 code is rejected (can be ignored)
2. Doesn't complete preprocessing (due to no Cura profile?) and the Print button is not enabled.

Looking ahead:

No ;LAYER:n tags in the GCODE. This can be fixed with changes in Slic3r settings.
The Slic3r profile is not recognized.

I will look into these if you want to assign it to me ...

[tomasf]

Compatibility with Slic3r would be good. We should have a mechanism for ignoring some codes, like G21, in TFPPrinter, at the dequeueCode level or something. If Slic3r has an embedded profile like Cura, it would be good to be able to read it, yeah.