Changed extrusion length to hidden setting

Pulled latest master from kliment/printun and changed extrusion length
to a hidden setting. It is now saved upon change as
settings.last_extrusion and can be set as such. Also added default
extrusion length that may be used as a fall back for deploying printrun
with a different default extrusion length without having to change that
everywhere in the code.

Made default extrusion length a setting

Added "extrusion_default" to settings and made it editable via settings
file. Setting is not written back when the user changes it through the
gui.

But in my world, you have nothing to fear.