WIP Extending the expressivity of ExtrusionRole

Co-authored-by: supermerill <merill@free.fr>
Based on the unfinished idea of @supermerill, defining ExtrusionRole
as a bit mask of ExtrusionRoleModifier.
Because the ExtrusionRole was used for both ExtrusionEntity and
G-code export / G-code viewer, the original ExtrusionRole had to be split
to ExtrusionRole and GCodeExtrusionRole to support bitmask attributes
for the former while keeing a low number of ordinary values for the latter.

src/libslic3r/Print.cpp

@@ -1029,7 +1029,7 @@ void Print::_make_skirt()
         ExtrusionLoop eloop(elrSkirt);
         eloop.paths.emplace_back(ExtrusionPath(
             ExtrusionPath(
-                erSkirt,
+                ExtrusionRole::Skirt,
                 (float)mm3_per_mm,         // this will be overridden at G-code export time
                 flow.width(),
 				(float)first_layer_height  // this will be overridden at G-code export time