Style: Replaces uses of 0/NULL by nullptr (C++11)

Using clang-tidy's `modernize-use-nullptr`.
https://clang.llvm.org/extra/clang-tidy/checks/modernize-use-nullptr.html

editor/plugins/skeleton_editor_plugin.h

@@ -49,7 +49,7 @@ class SkeletonEditor : public Node {
 		PhysicalBone *physical_bone;
 		Transform relative_rest; // Relative to skeleton node
 		BoneInfo() :
-				physical_bone(NULL) {}
+				physical_bone(nullptr) {}
 	};
 
 	Skeleton *skeleton;