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/packed_data_container.cpp

@@ -381,11 +381,6 @@ void PackedDataContainer::_bind_methods() {
 	ADD_PROPERTY(PropertyInfo(Variant::PACKED_BYTE_ARRAY, "__data__"), "_set_data", "_get_data");
 }
 
-PackedDataContainer::PackedDataContainer() {
-
-	datalen = 0;
-}
-
 //////////////////
 
 Variant PackedDataContainerRef::_iter_init(const Array &p_iter) {
@@ -429,6 +424,3 @@ int PackedDataContainerRef::size() const {
 
 	return from->_size(offset);
 };
-
-PackedDataContainerRef::PackedDataContainerRef() {
-}