* Use GCC8 toolchain
* Allow appimage to preload included libgcc_s and stdc++ if necessary.
* Need to add header for boost::noncopyable
* use const strings for exceptions.
* use right header dir for shim
* Only use swrast_dri from appimage if it's old or doesn't exist.
* added note about where the apprun script was source from, thanks @darealshinji
* For reprap (really Marlin) or Repetier flavors and a toolchange retract, produce G10 S1 instead of G10.
Also refactored comments (were getting eaten by fw retract)
* Fixed tests to check for correct retract gcode output.
* Better reference to scalar.
* Added config checks for if fw tool retracts are in place and firmware is repetier/marlin, disable the advanced option.
* Disable z lift if firmware retraction is enabled.
* With gcode comments on, indicates which extruder Slic3r thinks it is retracting/unretracting.
