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.

core/io/xml_parser.h

@@ -66,15 +66,15 @@ public:
 	};
 
 private:
-	char *data;
-	char *P;
-	uint64_t length;
+	char *data = nullptr;
+	char *P = nullptr;
+	uint64_t length = 0;
 	void unescape(String &p_str);
 	Vector<String> special_characters;
 	String node_name;
-	bool node_empty;
-	NodeType node_type;
-	uint64_t node_offset;
+	bool node_empty = false;
+	NodeType node_type = NODE_NONE;
+	uint64_t node_offset = 0;
 
 	struct Attribute {
 		String name;