Port member initialization from constructor to declaration (C++11)

Using `clang-tidy`'s `modernize-use-default-member-init` check and
manual review of the changes, and some extra manual changes that
`clang-tidy` failed to do.

Also went manually through all of `core` to find occurrences that
`clang-tidy` couldn't handle, especially all initializations done
in a constructor without using initializer lists.

editor/plugins/skeleton_3d_editor_plugin.h

@@ -46,10 +46,9 @@ class Skeleton3DEditor : public Node {
 	};
 
 	struct BoneInfo {
-		PhysicalBone3D *physical_bone;
+		PhysicalBone3D *physical_bone = nullptr;
 		Transform relative_rest; // Relative to skeleton node
-		BoneInfo() :
-				physical_bone(nullptr) {}
+		BoneInfo() {}
 	};
 
 	Skeleton3D *skeleton;