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/string_name.cpp

@@ -388,12 +388,6 @@ StringName StringName::search(const String &p_name) {
 	return StringName(); //does not exist
 }
 
-StringName::StringName() {
-
-	_data = nullptr;
-}
-
 StringName::~StringName() {
-
 	unref();
 }