Handle non existing STL files gracefully
Prevent crash when changing number of extruders
Fix the extruder override logic. All role-based extruder options now default to 0, meaning no override is defined and the default (inherited) extruder is used. The default extruder option does not use the shortcut logic anymore (it was thus moved to be a CLI-specific logic)
Specify that extruder indices are 1-based in tooltips
Merge pull request #11 from alexrj/master

Merge from master