and for separation of concerns:
The final G-code preview no more uses ExtrusionPaths structure
to hold the G-code path data extracted by parsing the G-code.
Instead, the ExtrusionPath class has been trimmed down back to
the original size before the G-code preview was introduced,
and a new GCodePreviewData::Extrusion::Path class was created to hold
the additional path data as the extruder ID, color change ID
and fan speed.
Iterator loops replaced with C++11 loops.
Fixed clone() methods to return an ExtrusionEntity*.
PerimeterGenerator now uses move semantics on ExtrusionEntity a little
bit more.
