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/pck_packer.cpp

@@ -180,11 +180,6 @@ Error PCKPacker::flush(bool p_verbose) {
 	return OK;
 };
 
-PCKPacker::PCKPacker() {
-
-	file = nullptr;
-};
-
 PCKPacker::~PCKPacker() {
 	if (file != nullptr) {
 		memdelete(file);