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/func_ref.cpp

@@ -94,6 +94,3 @@ void FuncRef::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("set_function", "name"), &FuncRef::set_function);
 	ClassDB::bind_method(D_METHOD("is_valid"), &FuncRef::is_valid);
 }
-
-FuncRef::FuncRef() {
-}