ENH: allow generate wipe tower when print ByObject

allow generate wipe tower when print ByObject if there is only one object

Change-Id: I199e9cab7bd7c963edf58e829c663bb91c12f4f8
(cherry picked from commit 7153c30800022f3afb9811c67f88ba28c9b25d20)

src/slic3r/GUI/PartPlate.hpp

@@ -292,6 +292,8 @@ public:
     //whether it is empty
     bool empty() { return obj_to_instance_set.empty(); }
 
+    int printable_instance_size();
+
     //whether it is has printable instances
     bool has_printable_instances();