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/os/dir_access.cpp

@@ -446,11 +446,3 @@ bool DirAccess::exists(String p_dir) {
 	memdelete(da);
 	return valid;
 }
-
-DirAccess::DirAccess() {
-
-	_access_type = ACCESS_FILESYSTEM;
-}
-
-DirAccess::~DirAccess() {
-}