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/reference.cpp

@@ -109,9 +109,6 @@ Reference::Reference() :
 	refcount_init.init();
 }
 
-Reference::~Reference() {
-}
-
 Variant WeakRef::get_ref() const {
 
 	if (ref.is_null())
@@ -138,9 +135,6 @@ void WeakRef::set_ref(const REF &p_ref) {
 	ref = p_ref.is_valid() ? p_ref->get_instance_id() : ObjectID();
 }
 
-WeakRef::WeakRef() {
-}
-
 void WeakRef::_bind_methods() {
 
 	ClassDB::bind_method(D_METHOD("get_ref"), &WeakRef::get_ref);