Add the skin_angles setting which is analogous to the infill_angles setting.

resources/definitions/fdmprinter.def.json

@@ -905,6 +905,14 @@
                     "value": "top_bottom_pattern",
                     "settable_per_mesh": true
                 },
+                "skin_angles":
+                {
+                    "label": "Top/Bottom Line Directions",
+                    "description": "A list of integer line directions to use when the top/bottom layers use the lines or zig zag pattern. Elements from the list are used sequentially as the layers progress and when the end of the list is reached, it starts at the beginning again. The list items are separated by commas and the whole list is contained in square brackets. Default is an empty list which means use the traditional default angles (45 and 135 degrees).",
+                    "type": "[int]",
+                    "default_value": "[ ]",
+                    "settable_per_mesh": true
+                },
                 "wall_0_inset":
                 {
                     "label": "Outer Wall Inset",