Implemented raft support for the wipe tower

https://github.com/prusa3d/Slic3r/issues/324

Implemented a correct layer height preview for the wipe tower layers,
if the wipe tower layer height is not constant due to the application
of raft.

xs/src/libslic3r/GCode/WipeTower.hpp

@@ -75,6 +75,7 @@ public:
 	{
 		// Print heigh of this tool change.
 		float					print_z;
+		float 					layer_height;
 		// G-code section to be directly included into the output G-code.
 		std::string				gcode;
 		// For path preview.