Split the external infill pattern to separate top / bottom config values.

Based on f9344a00e3
thanks @supermerill
Implements  #475, #479, #1133, #1474

src/libslic3r/Surface.hpp

@@ -98,6 +98,7 @@ public:
     bool is_solid() const;
     bool is_external() const;
     bool is_internal() const;
+    bool is_top() const;
     bool is_bottom() const;
     bool is_bridge() const;
 };