Changed handling of priming extrusions to allow injection of filament and toolchange custom gcodes

The priming extrusions were handled separately from the rest of the wipe tower toolchanges. In order to be able to use the logic from previous commit for them (custom toolchange gcodes etc), some unpleasant code shuffling was needed

src/libslic3r/GCode/WipeTower.hpp

@@ -119,6 +119,12 @@ public:
         // Is this a priming extrusion? (If so, the wipe tower rotation & translation will not be applied later)
         bool                    priming;
 
+        // Initial tool
+        int initial_tool;
+
+        // New tool
+        int new_tool;
+
 		// Sum the total length of the extrusion.
 		float total_extrusion_length_in_plane() {
 			float e_length = 0.f;
@@ -134,7 +140,7 @@ public:
 	};
 
 	// Returns gcode to prime the nozzles at the front edge of the print bed.
-	virtual ToolChangeResult prime(
+	virtual std::vector<ToolChangeResult> prime(
 		// print_z of the first layer.
 		float 						first_layer_height, 
 		// Extruder indices, in the order to be primed. The last extruder will later print the wipe tower brim, print brim and the object.