Refactoring of GCode::process_layer().

Refactoring of GCode export of color changes, extruder switches etc,
so that the "color change" like extruder switches are applied first
at the Wipe Tower / G-code export, so that adding / removing
an extruder switch at the G-code preview slider does not invalidate
slicing.

src/libslic3r/Extruder.hpp

@@ -48,9 +48,6 @@ public:
     double retract_length_toolchange() const;
     double retract_restart_extra_toolchange() const;
 
-    // Constructor for a key object, to be used by the stdlib search functions.
-    static Extruder key(unsigned int id) { return Extruder(id); }
-
 private:
     // Private constructor to create a key for a search in std::set.
     Extruder(unsigned int id) : m_id(id) {}