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/variant.cpp

@@ -3189,16 +3189,9 @@ Variant::Variant(const IP_Address &p_address) {
 
 Variant::Variant(const Variant &p_variant) {
 
-	type = NIL;
 	reference(p_variant);
 }
 
-/*
-Variant::~Variant() {
-
-	clear();
-}*/
-
 uint32_t Variant::hash() const {
 
 	switch (type) {